It's a cake made without baking powder, its extreme softness is obtained only by whisking the egg whites until stiff and the butter whisked until you have a cream.
The butter must be very soft, at room temperature. Knead in the mixer with a whisk, or in a bowl with electric beaters until you have a smooth texture (it takes 5-6 minutes, make sure to remove the butter from the sides of the bowl, mix and continue beating). Add the egg yolks one by one, by inserting the following only when the former is well incorporated. Pour gradually 250 g of sugar, continuing to whip at medium speed. Finally, add the flour and potato flour sifted together, by lowering the speed. Now put the grated lemon peel.
Whip the egg whites with a few drops of lemon juice, then add the remaining sugar (50 g). With a spatula pour them gently into the mixture obtained previously