Thoroughly butter the bottom and sides of a 9-inch round cake pan with 2-inch sides. Line the bottom with a round of parchment paper. Coat the sides of the pan with flour, shaking out the excess.

Make the cake: Sift the potato starch and baking powder into a bowl. In an electric mixer, cream the butter until smooth. Add the 1 cup sugar gradually, beating until well blended. Add the egg yolks one at a time, beating well after each addition and scraping down the sides of the bowl once or twice. Beat in the lemon zest, rosemary, and vanilla. By hand, stir in the dry ingredients just until blended.

Beat the egg whites and the remaining 1 tablespoon sugar to soft peaks. Fold half of the beaten whites into the potato starch mixture to lighten it, then gently fold in the remaining whites. Transfer the batter to the prepared pan, leveling it with a spatula.

Place in a cold oven and turn the thermostat to 350ºF. Bake until the cake is well risen, nicely browned, and beginning to pull away from the sides of the pan, 50 to 55 minutes. Let cool completely in the pan on a rack, then invert onto the rack and carefully remove the parchment paper. Invert again onto a serving platter. Put the confectioner's sugar in a sieve and dust the surface of the cake lightly with it.

Thirty minutes before serving, put the fruit in a bowl, add granulated sugar to taste, and crush the fruit with a fork until it is roughly mashed. Let it stand 30 minutes to render some juice.

Cut the cake into 12 slices. Serve a slice of cake with crushed fruit spooned over and around it and a dollop of whipped cream on the top or alongside.