- 10 ounces (1¼ cups) unsalted butter, at room temperature, plus more for the pan
- 1 pound rhubarb, cut into 4-inch pieces
- 1½ cups granulated sugar, divided
- ½ cup packed light brown sugar, divided
- 2 cups cake flour or all-purpose flour
- 1¼ teaspoons baking powder
- ½ teaspoon fine salt
- 1 teaspoon grated lemon zest (from 1 lemon)
- 1 teaspoon pure vanilla extract
- 4 large eggs, at room temperature
- ⅓ cup full-fat plain yogurt
- Vanilla or strawberry ice cream, for serving
- Preheat the oven to 325°F. Butter and line an 8-inch square pan with parchment. Butter the parchment.
- Toss the rhubarb, ½ cup of the granulated sugar, and ¼ cup of the brown sugar together in a bowl. Layer the rhubarb across the bottom of the pan in a grid pattern, adding any remaining sugar mixture to the pan.
- Whisk together the flour, baking powder, and salt. In a separate bowl, beat the butter, remaining sugars, and lemon zest with an electric mixer until light and fluffy, 4 minutes. Scrape down the sides of the bowl. Add the vanilla and eggs, one at a time, beating after each addition.
- Stir in the yogurt on low speed. (The mixture may look curdled.) Gradually add the flour mixture and stir to combine.
- Spoon the batter over the rhubarb and spread evenly. (The pan will be almost full.)
- Bake until a toothpick inserted in the center comes out clean, 1 hour, 15 minutes. Cool on a wire rack for 15 minutes, then unmold onto a platter. (Discard the parchment paper.) Cut the cake into squares and serve warm with ice cream.