Cream butter and sugar until smooth. Beat egg yolks into mixture, then add the sifted flour and milk little by little, beating well. Add baking powder and salt; then fold in finely chopped apple. Beat egg whites until stiff and fold them gently into the mixture. Form the mixture into small balls the size of a walnut; drop them into boiling fat, and fry until golden. Drain, sprinkle with icing sugar and serve immediately.