Sourdough Rhubarb Cake with Brown Sugar Crumble
This sourdough rhubarb cake is impossibly moist, perfectly tangy, and topped with a buttery brown sugar crumble. The best rhubarb cake recipe for using up your sourdough starter discard.
The Sourdough Rhubarb Cake You Will Make Every Single Spring
If you have a jar of sourdough discard sitting on your counter and an armful of rhubarb stalks from the garden, this is exactly where they belong. This sourdough rhubarb cake is tender, deeply flavored, and crowned with a crackly brown sugar oat crumble that will have everyone reaching for a second slice before the first one is even finished.
The secret here is the sourdough starter discard. It does not make this cake taste sour or bread-like. What it does is add a gentle, almost imperceptible tang that plays beautifully against the bright tartness of the rhubarb, and it keeps the crumb extraordinarily moist for days. Think of it as a flavor amplifier with a bonus side effect of using up that discard you would otherwise throw away.
This is the kind of cake that works as a casual afternoon snack with coffee, a relaxed weekend brunch centerpiece, or a proper dessert with a scoop of vanilla ice cream. It is unfussy, forgiving, and completely irresistible.
Why This Is the Best Rhubarb Cake Recipe for Sourdough Bakers
Most rhubarb cake recipes are good. This one is memorable. Here is what sets it apart:
- The sourdough discard adds complexity that plain yogurt or buttermilk simply cannot replicate.
- The brown sugar crumble creates a gorgeous caramelized crust that contrasts with the soft, jammy rhubarb pockets inside.
- No special equipment is needed beyond a standard cake pan and a hand mixer.
- It works beautifully as a sourdough rhubarb bread too, baked in a loaf pan for a more sliceable format.
Chef's Tip: Do not skip chilling the crumble topping while you mix the batter. Cold butter in the crumble is what gives you those satisfying big, crunchy clusters on top rather than a fine, sandy coating.
Choosing and Prepping Your Rhubarb
For the best flavor, look for firm, brightly colored stalks with a deep red or pink hue. The redder the rhubarb, the more visually stunning your cake will be, though green rhubarb tastes just as good. Always discard the leaves entirely as they are toxic and should never be consumed.
Chop your rhubarb into roughly half-inch pieces. You want chunks small enough to distribute through the batter evenly but large enough to hold their shape during baking and give you little bursts of tart fruit in every bite. If you are using frozen rhubarb, thaw it completely and press it between paper towels until it is as dry as possible.
If you love the classic pairing of strawberry and rhubarb, this recipe adapts beautifully. Swap out one cup of rhubarb for fresh halved strawberries for a gorgeous sourdough strawberry rhubarb version that tastes like summer in cake form.
Having the right tools makes baking feel effortless rather than stressful. A good hand mixer, a reliable cake pan with even heat distribution, and quality vanilla extract genuinely make a difference in the final result.
How to Make Rhubarb Cake with Sourdough Starter
This recipe follows a simple, classic cake method with a few important moments worth slowing down for.
Cream your butter and sugar properly. Give it a full two to three minutes until the mixture is noticeably pale and fluffy. This step builds the structure of the cake.
Do not panic when the batter looks curdled. Once you add the sourdough starter discard, the batter may appear a little lumpy or broken. Keep going. It comes together beautifully once the flour is folded in.
Fold, do not stir. Once the flour goes in, switch to a rubber spatula and use gentle folding motions. Overmixing develops gluten and turns your cake dense and tough.
Chef's Tip: A toothpick inserted near a piece of rhubarb may come out with pink juice even when the cake is fully baked. Test a spot in the center of the batter rather than near a rhubarb chunk for an accurate reading.
Ready to dive in? Here is the complete step-by-step recipe:
Sourdough Rhubarb Cake with Brown Sugar Crumble
This sourdough rhubarb cake is impossibly moist, perfectly tangy, and topped with a buttery brown sugar crumble. The best rhubarb cake recipe for using up your sourdough starter discard.
Ingredients
- 2 cups all-purpose flour, spooned and leveled
- 1 1/2 tsp baking powder
- 1/2 tsp baking soda
- 1/2 tsp fine sea salt
- 1 tsp ground cinnamon
- 1/2 cups unsalted butter, softened to room temperature
- 3/4 cups granulated sugar
- 2 large eggs, room temperature
- 3/4 cups sourdough starter discard, unfed, at room temperature
- 1/4 cups whole milk
- 1 tsp pure vanilla extract
- 2 1/2 cups fresh rhubarb, trimmed and cut into 0.5-inch pieces
- 1/2 cups all-purpose flour, for crumble topping
- 1/2 cups brown sugar, packed, for crumble topping
- 3 tbsp unsalted butter, cold and cubed, for crumble topping
- 3 tbsp rolled oats, for crumble topping
Instruction
Preheat your oven to 350 degrees F (175 degrees C). Grease a 9-inch round cake pan or an 8x8-inch square baking dish and line the bottom with parchment paper.
Make the crumble topping first: In a small bowl, combine the 0.5 cup flour, brown sugar, and rolled oats. Add the cold cubed butter and use your fingertips to rub everything together until the mixture resembles coarse, clumpy sand. Refrigerate while you prepare the batter.
In a medium bowl, whisk together the 2 cups flour, baking powder, baking soda, salt, and cinnamon. Set aside.
In a large bowl, beat the softened butter and granulated sugar together with a hand mixer or stand mixer on medium speed for 2 to 3 minutes, until light and fluffy.
Add the eggs one at a time, beating well after each addition. Mix in the vanilla extract.
Add the sourdough starter discard and milk to the butter mixture and stir gently to combine. The batter may look slightly curdled at this stage but that is completely normal.
Fold the dry ingredients into the wet ingredients using a rubber spatula until just combined. Do not overmix.
Gently fold in the rhubarb pieces, distributing them evenly throughout the batter.
Pour the batter into the prepared pan and spread it into an even layer. Scatter the chilled crumble topping evenly over the surface.
Bake for 45 to 50 minutes, until the top is deep golden brown and a toothpick inserted into the center of the cake comes out clean or with just a few moist crumbs.
Allow the cake to cool in the pan for at least 20 minutes before slicing and serving.
Equipment
- 9-inch round cake pan or 8x8-inch baking dish
- Parchment paper
- Large mixing bowl
- Medium mixing bowl
- Small mixing bowl
- Hand mixer or stand mixer
- Rubber spatula
- Whisk
- Chef's knife and cutting board
Notes
Serving, Storing, and Variations
This cake is wonderful slightly warm with a dusting of powdered sugar, or at room temperature the next day when the crumb has had time to settle and the flavors have deepened. For dessert, serve it alongside a generous scoop of vanilla bean ice cream or a dollop of lightly sweetened whipped cream.
Storage: Cover and keep at room temperature for up to 2 days, or refrigerate for up to 5 days. Wrap individual slices tightly and freeze for up to 2 months.
Loaf variation: To make sourdough rhubarb bread, pour the batter into a greased 9x5-inch loaf pan and bake at 350 degrees F for 55 to 65 minutes. The result is a sliceable, snackable loaf that is perfect for morning toast.
However you choose to serve it, this sourdough rhubarb cake recipe is one you will return to every spring without hesitation.
