In a bowl, cream the butter and sugar; beat in egg yolks and lemon zest. Combine the flour, baking soda, baking powder and salt; add to creamed mixture alternately with sour cream. Stir in chocolate chips and walnuts. In another bowl, beat egg white until soft peaks form; fold into the batter.
Pour into a greased 6-cup fluted tube pan. Bake at 350° for 35-40 minutes or until a toothpick inserted in the center comes out clean. Cool for 10 minutes before removing from pan to a wire rack.
In a small saucepan, combine sugar and juices. Bring to a boil. Brush over warm cake. Cool completely.