adapted from williams-sonoma
1 lb. plums, halved and pitted
1 1/4 cups granulated sugar
14 Tbs. (1 3/4 sticks) unsalted butter, at
4 eggs, separated
Finely grated zest of 1 orange
Finely grated zest of 1/2 lemon
1 tsp. vanilla extract
1 1/2 cups all-purpose flour
1/2 cup cornstarch
2 tsp. baking powder
1 Tbs. confectioners’ sugar
Preheat an oven to 375°F. Butter a 9-inch springform pan and dust with flour, tapping out the excess.
Place the plum halves in a bowl and sprinkle with 1/4 cup of the granulated sugar. Set aside.
In a large bowl, using an electric mixer set on high speed, beat together the butter and the remaining 1 cup granulated sugar until light and fluffy. Beat in the egg yolks one at a time, beating well after each addition. Stir in the orange zest, lemon zest and vanilla.
In a bowl, sift together the flour, cornstarch and baking powder. In another bowl, using the electric mixer fitted with clean beaters, beat the egg whites until stiff but not dry.
Fold the flour mixture into the butter mixture in 3 additions, alternating with the egg whites and ending with the whites. Do not overmix. Spread half of the batter in the prepared pan.
Bake for 10 minutes. Remove the pan from the oven and arrange the plum halves, cut sides up, on top of the prebaked batter. Carefully spread the remaining batter evenly over the plums. Return to the oven and bake until the cake is lightly browned on top, 50 to 55 minutes. Transfer to a wire rack and let cool for 5 minutes, then run a knife around the sides of the cake to loosen them from the pan.
Let cool for 45 minutes more, then remove the pan sides. Using a small sieve, dust the top of the cake with the confectioners’ sugar (I forgot to do this, and it was fine). Serve slightly warm or at room temperature.
Serves 8 to 10.