Preheat oven at 200 degree for 10 minutes before baking. Prepare a round shaped cake tin for baking.Beat the egg whites separately till it forms a soft peak. Add vanilla essence and sugar to this and keep it aside. Sieve all purpose flour and baking powder. Beat well the butter and powdered sugar. Add egg yolks one by one to this and mix well. Add the lemon and orange rinds. Add flour and milk alternatively to this and beat well. Add egg white mixture to this and fold in. Pour the batter to the prepared cake tin and bake at 200 degree for first 10 minutes and 180 degree for another 25 - 30 minutes.