Ingredients
8 ounces rhubarb (about 3 stalks), halved lengthwise and cut into 3-inch pieces
1 cup granulated sugar
1 3/4 cups unbleached all-purpose flour, plus more for pan
1/2 cup light-brown sugar
3/4 teaspoon kosher salt
4 tablespoons cold unsalted butter, cut into small pieces, plus 6 tablespoons room-temperature butter, and more for pan
1 1/4 teaspoons baking powder
2 large eggs, room temperature
1/4 cup sour cream, room temperature
1 teaspoon grated orange zest
1 cup heavy cream
Preparation
Preheat oven to 350 degrees. Toss rhubarb with 1/4 cup sugar. Let stand, tossing occasionally, until sugar dissolves, about 30 minutes. To make streusel, stir together 1/2 cup flour, brown sugar, and 1/4 teaspoon salt. Using your fingers, cut in cold butter until medium clumps form. Press mixture into bottom of bowl; refrigerate until ready to use, at least 20 minutes.
Butter a 9-inch round cake pan and line bottom with parchment; butter parchment. Dust pan with flour; tap out excess. Whisk together baking powder and remaining 1 1/4 cups flour and 1/2 teaspoon salt. In another bowl, stir together room-temperature butter and remaining 3/4 cup sugar. Whisk in eggs, one at a time. Fold in half of flour mixture, then sour cream and zest. Stir in remaining flour mixture just to combine. Spread batter in bottom of prepared pan.
Break streusel into large pieces; sprinkle half over batter. Strain rhubarb (reserving syrup); scatter over batter, then top with rest of streusel. Bake until cake pulls away from sides of pan and a tester inserted in center comes out clean, 40 to 45 minutes. Let cool 20 minutes. Invert onto a wire rack; let cool completely. Whip cream to soft peaks; sweeten to taste with syrup. Whisk until soft peaks return and serve with cake.
