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 and butter the paper. Coat the pan with flour, shaking out the excess.
Beat the egg whites with 2 tablespoons sugar until it forms soft peaks; set aside in the refrigerator. Sift the potato starch and baking powder into a bowl.
In an electric mixer, cream the butter with a pinch of salt until smooth. Add remaining 1 cup sugar gradually, beating until well blended. Add the egg yolks 1 at a time, beating well after each addition and scraping down the sides of the bowl once or twice. Beat in the lemon zest and vanilla. Add the sifted potato starch and baking powder all at once. Mix until just blended. Fold half of the beaten whites into the batter 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 then preheat to 350 degrees F. Bake until the cake is well raised, 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 lightly dust the surface of the cake
Most supermarkets stock potato starch with the baking ingredients.