Preheat the oven to 325F and grease and flour a square or round 10-inch springform cake pan. Separate the eggs and place the yolks in a large mixing bowl. Add the superfine sugar, vanilla sugar, and lemon zest, and beat together until thick and creamy.

In a separate bowl, whisk the egg whites until stiff peaks form. In a separate bowl, sift the cake meal with the potato flour (twice if possible) and stir in the ground almonds. Fold the dry ingredients into the egg yolk mixture, along with the orange and lemon juice, and then carefully fold in the egg whites.

Pour into your prepared cake pan and bake for 1. hours. When cool, dust with confectioners’ sugar.