The butter mix with powdered sugar, add egg yolks, vanilla and flour with baking powder. Knead into a soft dough. Attention is a ball of it (1/3) and put in the freezer. Whip with granulated sugar and the feta cheese. In a buttered and dusted with flour pan put the first batter and pour it on the egg white mixture. Top grate the dough from the freezer, sprinkle with the walnuts and bake in a moderate oven.