Separate the yolks from the whites into two different bowls. Beat the egg whites until stiff, then beat the yolks with the sugar until you get a foamy mixture.
Add the potato starch and vanilla baking powder, previously sifted, to the yolks. Gradually pour in the milk in a thin stream.
Take the lemon and grate the zest into the mixture. Cut it in half and squeeze the juice, mixing it well into the batter.
Gently fold in the beaten egg whites, mixing from the bottom up until you obtain a smooth batter.
Pour the mixture into a buttered and floured baking tin.
Bake at 180 °C for about 30 minutes.
Remove the cake from the oven and serve with a dusting of impalpable sugar.