Whisk the egg whites, beat with granulated sugar until stiff. Sift flour with baking powder.
Cream butter with icing sugar, vanilla sugar and salt. Stir in the egg yolks bit by bit. Sift flour with baking powder.
Alternately fold flour and baking powder mixture and beaten egg white into the mixture.
Divide the mass. Stir the lemon peel into one half, cocoa and oil into the other.
Grease and flour the mould, filling in the masses alternately. Carefully prick patterns into the dough with a fork.
Bake at 180° for about 1 hour. When cool, turn out and sprinkle with icing sugar.