Rhubarb is tart, dramatic, and deeply underrated — and this dump cake is its best vehicle. You get a bubbling, jammy rhubarb base with a golden, crumbly cake topping that crisps at the edges and stays tender in the center. No mixer. No creaming. No fuss. What makes this version different? A cornstarch-thickened base (no watery pools!), a from-scratch cake topping that skips the boxed mix entirely, and one pro tip that changes everything: dot cold butter on top rather than melting it all — half the butter melts into the mix, the other half creates crackly, uneven crispy patches.
Yield: 12 servings | Pan: 23 × 33 cm (9 × 13 inch) | Bake time: 45–50 minutes
Why This Recipe Works
Rhubarb sits at a pH of just above 3 — close to lemon juice — because of its high oxalic and malic acid content. That acidity is what gives this dessert its signature brightness, but it also releases a lot of liquid during baking. If left unchecked, that liquid turns your cake layer soggy before it has a chance to set.
Two things fix this: cornstarch in the filling binds the released juice into a glossy, thick compote rather than a runny puddle, and cold butter dotted (not fully poured) on top keeps the dry flour layer from absorbing moisture too quickly. The result? A bottom that thickens like a fruit pie filling, and a top that bakes like a golden crumble.
Did you know? Rhubarb’s acidity also helps the baking powder in the topping activate more efficiently — the acid-leavening interaction produces extra lift in the cake layer without you adding a single extra gram of baking powder.
Ingredients
Rhubarb Filling
| Ingredient | Grams | Volume | Purpose |
|---|---|---|---|
| Fresh or frozen rhubarb, chopped ½ inch | 540 g | 6 cups | Tart fruit base |
| Granulated white sugar | 200 g | 1 cup | Sweetens and draws out juice |
| Cornstarch | 30 g | ¼ cup | Thickens released juices |
| Fresh lemon juice | 15 ml | 1 tbsp | Brightens flavor, adds acid balance |
| Vanilla extract | 5 ml | 1 tsp | Rounds out tartness |
| Ground cinnamon | 1 g | ¼ tsp | Warm depth (optional but tested) |
From-Scratch Cake Topping
| Ingredient | Grams | Volume | Purpose |
|---|---|---|---|
| All-purpose flour | 300 g | 2½ cups | Structure |
| Granulated sugar | 150 g | ¾ cup | Sweetness + browning |
| Baking powder | 6 g | 1½ tsp | Lift and texture |
| Fine salt | 3 g | ½ tsp | Flavor contrast |
| Cold salted butter, cubed | 170 g | ¾ cup | Fat, flakiness, crunch |
| Coarse sugar (for topping) | 15 g | 1 tbsp | Crackly crust finish |
Testing note: I tested this with both boxed vanilla cake mix and from-scratch. The from-scratch version gives you control over sweetness and produces a more tender, less artificial-tasting topping. If you’re in a rush, a 432 g (15.25 oz) box of vanilla cake mix with 170 g (¾ cup) melted butter works — but reduce to ½ cup (113 g) butter if using the boxed route, or the top gets greasy.
Equipment
Required:
- 23 × 33 cm (9 × 13 inch) baking dish (glass or ceramic preferred for even heat)
- Kitchen scale
- Mixing bowl
- Pastry cutter or two forks (for cutting in butter)
- Oven thermometer
Nice to have:
- Parchment paper (for easy cleanup of drips)
- Bench scraper
- Rubber spatula for spreading filling
Step-by-Step Instructions
Step 1: Prep and Preheat (5 minutes)
Preheat your oven to 175°C (350°F) with the rack in the center position. Allow 20 minutes of full preheat — an under-heated oven is the #1 reason dump cakes bake unevenly. Lightly grease the baking dish.
Visual cue: Oven should be at full temperature before the dish goes in. Use an oven thermometer; many home ovens run 10–15°C cooler than displayed.
Step 2: Make the Filling (5 minutes)
Add chopped rhubarb to the greased baking dish. Sprinkle sugar, cornstarch, cinnamon, vanilla, and lemon juice directly over the rhubarb. Toss gently with a spatula until coated, then spread into an even layer.
Visual cue: The rhubarb should look lightly dusty from the cornstarch — not wet or syrupy yet. That’s correct. It will release its juice and gel during baking.
Avoid this: Don’t let the filled dish sit for more than 10 minutes before baking. Extended resting causes premature juice release, which can soak the flour topping before it sets.
Step 3: Make the Cake Topping (7 minutes)
In a bowl, whisk together flour, sugar, baking powder, and salt. Add cold cubed butter and work it in with a pastry cutter or your fingertips until the mixture resembles coarse, pebbly crumbs — think rough sand with pea-sized butter pieces throughout.
Visual cue: You want visible pale yellow butter chunks, not a smooth paste. Those chunks melt in the oven into rich, crumbly pockets.
Step 4: Assemble and Bake (45–50 minutes)
Scatter the crumb topping evenly over the rhubarb filling. Do not press it down. Sprinkle coarse sugar over the top.
Bake at 175°C (350°F) for 45–50 minutes, until:
- The top is deep golden brown (not pale tan)
- The edges are visibly bubbling and thick
- A skewer inserted into the topping comes out with moist crumbs — not raw dough
Visual cue: At 35 minutes, the filling will bubble up the sides before the topping fully sets — this is normal. Do not open the oven before the 35-minute mark.
Pro tip: If the top is browning too fast before the filling bubbles, tent loosely with foil at the 30-minute mark and remove it for the final 10 minutes.
Step 5: Rest and Serve
Cool in the pan for at least 20 minutes before serving. The filling continues to thicken as it cools.
Visual cue: Straight from the oven, the filling will look watery if you poke through. After resting, it should be thick and jammy — like pie filling.
Troubleshooting
| Problem | Likely Cause | Fix |
|---|---|---|
| Watery, soupy bottom | Not enough cornstarch, frozen rhubarb not drained | Increase cornstarch to 40 g; if using frozen, pat dry first |
| Pale, powdery dry patches on top | Butter not distributed evenly | Ensure butter is evenly spread; add 1–2 tbsp extra melted butter to dry spots |
| Soggy cake layer | Rested too long before baking, or oven too low | Bake immediately after assembling; verify oven temp with thermometer |
| Overly tart filling | Under-ripe or early-spring rhubarb | Increase sugar by 25–50 g or add 15 g (1 tbsp) honey to filling |
| Burned edges, raw center | Oven rack too high or glass dish | Move rack to center; glass dishes retain heat — reduce temp by 5°C if browning too fast |
| Dense, heavy topping | Butter was too warm/melted | Use cold butter straight from fridge; don’t over-work the crumb |
Substitutions & Variations
Dairy-free: Replace butter with solid coconut oil (same weight). The topping will be slightly less crispy but still delicious. Avoid liquid oils — they make the topping dense and greasy.
Gluten-free: Swap all-purpose flour 1:1 with a GF baking blend that contains xanthan gum. Add an extra 5 g (1 tsp) cornstarch to compensate for reduced binding.
Strawberry-rhubarb version: Replace 180 g (2 cups) rhubarb with fresh hulled strawberries. Reduce sugar in the filling by 25 g since strawberries are sweeter. Tested and loved — brighter color, slightly sweeter finish.
Spiced apple-rhubarb: Replace 2 cups rhubarb with diced Granny Smith apple. Add ½ tsp nutmeg to the topping. A cozy fall variation.
Scaling to an 8-inch square pan: Halve all ingredients exactly. Reduce bake time to 35–40 minutes.
Storage, Make-Ahead & Freezing
Counter: Cover loosely and store at room temperature for up to 2 days. The topping softens slightly by day 2 — still tasty, less crispy.
Fridge: Covered tightly, up to 5 days. Bring to room temperature for 30 minutes before serving, or warm individual portions at 150°C (300°F) for 10 minutes to re-crisp the top.
Freezer: Freeze cooled, fully baked dump cake in an airtight container for up to 2 months. Thaw overnight in the fridge, then reheat uncovered at 175°C (350°F) for 15–20 minutes to restore some topping crunch.
What not to freeze: Don’t freeze the assembled-but-unbaked dump cake. The filling releases water during thawing and turns the dry topping into a dense, gummy paste.
Make-ahead tip: Mix and refrigerate the dry topping (flour + sugar + baking powder + salt, without butter) up to 3 days ahead. Cut in cold butter just before assembling and baking.
Serving Suggestions
Serve warm with a scoop of vanilla bean ice cream — the cold cream melting into the hot, jammy rhubarb filling is the whole point. For something lighter, try a dollop of lightly sweetened whipped cream or a swirl of crème fraîche, which echoes rhubarb’s tanginess beautifully. A drizzle of warm custard sauce also works wonderfully if you’re serving this as a dinner party dessert.
FAQs
Can I use frozen rhubarb?
Yes — but don’t thaw it first. Thawed rhubarb turns mushy and releases too much water before baking. Use straight from frozen, and increase cornstarch by 5–10 g to account for the extra moisture.
Why is my topping still white and powdery after baking?
This means the butter didn’t distribute evenly enough to hydrate all the flour. Drizzle 1–2 tbsp of melted butter over dry patches before baking, or press the crumble down very gently in those spots.
Can I double this recipe?
Yes — double all ingredients and bake in two 9 × 13 pans simultaneously. Rotate pans halfway through. Don’t try to bake one oversized pan — the depth will cause uneven cooking.
Do I need to peel rhubarb before using it?
No — rhubarb skin softens fully during baking and adds color to the filling. Do remove and discard the leaves; rhubarb leaves contain high levels of oxalic acid and are toxic.
Can I bake this in an OTG?
Absolutely. Use the lower heating element with convection mode off. Bake at the same temperature (175°C/350°F) on the center rack. Check at 40 minutes; OTGs often run hot at the top, so tent with foil early if needed.
Can I add Jell-O or gelatin to the filling?
Yes — this is a popular variation. One 85 g (3 oz) packet of strawberry or cherry gelatin sprinkled over the rhubarb adds sweetness and a gorgeous ruby color. Skip 50 g of the sugar if you add it, as the gelatin is already sweet.
Nutrition & Disclaimer
Approximate per serving (based on 12 portions): ~310 calories | 14 g fat | 45 g carbs | 3 g protein | 2 g fiber
Values are estimates based on standard ingredient brands and portion sizes. Adjust for specific brands, butter fat content, and serving size. Always follow food safety best practices: refrigerate leftovers within 2 hours of serving, and never consume rhubarb leaves.
Final Thoughts
This rhubarb dump cake is proof that simple doesn’t mean boring. The layering method, the cornstarch trick, and the cold-butter crumble are small decisions that add up to a dessert that tastes genuinely considered — not thrown together. If you’re new to baking with rhubarb, this is the ideal starting point: forgiving, fast, and deeply satisfying. Make it once and it’ll become a spring ritual.
