Whip butter in the mixer and gradually add sugar until mixture is creamy. Add eggs one by one, beating well after each addition.
Lower speed and gradually add sifted flour and baking powder, alternating with the milk.
Add in vanilla or lemon or orange peel and mix.
Pour batter into a previously greased and floured 10 inch tube cake pan or a loaf pan.
Bake in preheated oven to 350°F (180°C) for 40 minutes or until cake tester comes out clean. Remove from the oven, cool on rack and invert cake.
Dust confectioner´s sugar over cake.
To prepare a Marble cake, remove 4 tablespoons flour and the lemon or orange peel.
Divide batter in half and stir 4 heaped tablespoons of sifted cocoa to one half.