Cream together butter, lemon peel, vanilla, 1/2 C. sugar and egg yolks. Add milk slowly. Beat egg whites until stiff. Add 1/2 C. sugar. Add flour and baking powder to creamed mixture. Fold in stiffly beaten egg whites. Turn batter into mold. Bake about 55 minutes at 350. For variety, add 1/2 C. freshly ground poppy seeds or 1/2 C. rum-soaked raisins.