1/2 cup (1 stick) Unsalted Butter, room temperature, plus more for the pan
2 cups All-Purpose Flour
1 tsp Baking Powder
1 tsp Kosher Salt
½ tsp Baking Soda
1 cup Granulated Sugar
4 Eggs, room temperature
2 tsp Pure Vanilla Extract
2 tsp Lemon Zest, finely grated
1 cup Sour Cream
Confectioners’ sugar and whipped cream for serving (optional)
Position a rack in the center of the oven and preheat oven to 350 degrees. Lightly grease a 9-inch-by-13-inch baking dish.
Pour Rhubarb and strawberry sauce into a bowl; mash well with a potato masher or fork. Set aside.
In a medium bowl, whisk together flour, baking powder, salt and baking soda until combined.
Using an electric mixer, cream butter and sugar on medium-high speed until light and fluffy. Add eggs, one at a time, and beat until combined. Add vanilla extract and lemon zest and beat on low to combine.
With the mixer on low, add flour mixture in three parts, alternating with the sour cream and beginning and ending with flour. Pour two-thirds of the batter into the prepared pan and smooth with a spatula. Using a spoon, add half of the reserved rhubarb and strawberry sauce in small dollops. Run a wooden skewer or table knife horizontally and then vertically in a swirling motion to create a marbled effect. Top cake with alternating spoonfuls of remaining batter and rhubarb; repeat swirling motions.
Bake until a skewer inserted into the center comes out clean, about 50 minutes. Let cool on a rack at least 30 minutes. Serve dusted with confectioners’ sugar and lightly sweetened whipped cream, if desired.
This recipe can be halved to fit an 8-inch square or round baking pan. (In that case, line the pan with buttered parchment paper and invert the cake onto a cooling rack after letting it cool for 10 minutes.)
The cake can be served both warm and completely cooled.