Moist, tender, and dotted with soft pink rhubarb, this German-style sheet cake is the kind of dessert that disappears fast once it hits the table. The cake stays plush from the buttermilk batter, while the rhubarb gives each bite a bright little burst that keeps it from feeling heavy. A dusting of powdered sugar on top is all it needs.
What makes this version work is the balance. The rhubarb gets a short sugar rest first, which softens its sharp edges without turning it mushy, and the buttermilk reacts with the baking soda to give the crumb a light lift. You end up with a cake that bakes up even, sliceable, and moist all the way through instead of dense around the fruit.
If you’ve ever had rhubarb cake that baked up soggy in the middle or tasted flat, the small details here matter. Below I’ve added the one trick that keeps the fruit from sinking, plus a few smart swaps and storage notes for when you want to bake it ahead.
The rhubarb softened just enough in the sugar, and the cake baked up incredibly moist without getting gummy around the fruit. I used the powdered sugar on top and it made each slice look like something from a bakery.
Save this Oma’s Rhubarb Cake for the days when you want a tender German sheet cake with bright rhubarb and a simple powdered sugar finish.
The Reason the Rhubarb Stays Bright Instead of Disappearing Into the Crumb
Rhubarb cakes can go wrong in two opposite ways: the fruit turns stringy and sharp, or it melts into the batter and leaves you with a wet center. The short sugar rest helps pull out a little juice, which softens the rhubarb without breaking it down completely. That matters because you want the fruit to hold its shape long enough to bake into the cake instead of dissolving into it.
The other detail that does the heavy lifting is the batter texture. It should be thick enough to suspend the rhubarb pieces, but not stiff. If the batter looks loose, the fruit sinks; if it’s overmixed, the crumb gets tough. Stop mixing as soon as the flour disappears, then fold in the rhubarb gently so the cake bakes up tender and even.
What Each Ingredient Is Actually Doing in This Cake
- Rhubarb — Fresh rhubarb gives the cake its tart, clean bite. Dice it evenly so it softens at the same rate; big chunks can stay too firm while smaller ones vanish into the crumb.
- Buttermilk — This is what gives the cake its soft, slightly tangy crumb. It also reacts with the baking soda, which helps the cake rise without tasting overly sweet. Plain milk works in a pinch, but the texture won’t be quite as tender.
- Butter and sugar — Creaming them together builds the base of the cake and traps air for lift. Don’t rush this step; you want the mixture pale and fluffy, not just combined.
- Baking soda — This is matched to the buttermilk, and the two need each other. If you swap in regular milk, the cake will lose some of that lightness unless you add an acid.
- Powdered sugar — It’s not just decoration. The light dusting finishes the cake with a clean look and a little extra sweetness that balances the tart rhubarb.
How to Build the Batter So the Cake Stays Tender
Softening the Rhubarb First
Toss the diced rhubarb with sugar and let it sit for about 15 minutes. You’ll see juice collect at the bottom of the bowl, and that’s exactly what you want. This step takes some of the harsh edge off the rhubarb and helps it bake into the cake without tasting raw. Don’t let it sit much longer or it can get too wet and make the batter heavy.
Mixing the Cake Base
Cream the butter and sugar until the mixture looks pale and fluffy, then beat in the eggs and vanilla. The batter should look smooth before you add the dry ingredients. If it looks curdled after the eggs go in, keep mixing for another few seconds; that usually smooths out once the flour and buttermilk are added. Once the dry ingredients go in, mix only until combined so the crumb stays soft.
Folding and Baking
Add the flour mixture and buttermilk in alternating additions, then fold in the sugared rhubarb gently. The batter should be thick and scoopable. Spread it into the pan and bake until the top is set and a toothpick comes out clean from the center, usually around 40 to 45 minutes. If the top browns before the middle is done, tent it loosely with foil for the last few minutes.
How to Adapt This Rhubarb Cake for Different Kitchens
Gluten-Free Version
Use a good 1:1 gluten-free baking flour in place of the all-purpose flour. The cake will still be tender, but it may be a little more fragile when warm, so let it cool completely before slicing. If your blend already contains xanthan gum, don’t add extra.
Dairy-Free Adjustment
Swap the butter for a dairy-free stick butter and use plain unsweetened plant milk with 1 tablespoon lemon juice stirred in for the buttermilk. The texture will still be soft, but the flavor loses a little of the classic tang that buttermilk brings.
Extra-Tart Rhubarb Cake
If your rhubarb is especially sharp, increase the sugar on the rhubarb by another 2 to 3 tablespoons. That keeps the cake balanced without making it cloying, and it works better than piling on more sugar to the batter itself.
Storage and Reheating
- Refrigerator: Store covered for up to 4 days. The cake stays moist, though the rhubarb will soften a bit more by day two.
- Freezer: Freeze individual slices tightly wrapped for up to 2 months. Thaw in the refrigerator so the crumb doesn’t turn soggy from condensation.
- Reheating: Warm slices briefly at room temperature or for a few seconds in the microwave. Don’t heat it too long or the cake dries out and the rhubarb turns jammy.
Answers to the Questions Worth Asking
Oma's Rhubarb Cake
Ingredients
Equipment
Method
- Toss the diced rhubarb with 1/2 cup sugar and set aside to macerate for 15 minutes.
- Preheat the oven to 350°F and grease a 9x13-inch baking pan.
- Cream together the softened butter and 1 1/2 cups sugar until light and fluffy.
- Beat in the eggs one at a time, then mix in the vanilla extract.
- Combine the flour, baking soda, and salt in a separate bowl.
- Add the dry ingredients to the butter mixture alternating with the buttermilk, mixing until just combined.
- Fold in the sugared rhubarb, then pour the batter into the prepared pan.
- Bake at 350°F for 40 to 45 minutes, until a toothpick inserted in the center comes out clean.
- Cool completely for 30 minutes, then dust the top with powdered sugar.
- Serve the cake after it is fully cooled.