Chocolate Chip Butter Cake

8 ounces butter
2 cups sugar
4 eggs, separated
2 2/3 cups sifted cake flour
2 teaspoons baking powder
1/2 teaspoon salt
1 cup milk
2 teaspoons vanilla
5 ounces German sweet chocolate

Cream butter and sugar together. Add yolks one at a time, beating well after each addition. Add sifted dry ingredients, alternating with milk and vanilla. Add in thirds, beating until smooth after each addition. Grate chocolate coarsely, reserving 1/3 cup for the topping. Beat whites until stiff and fold into batter, add remaining chocolate to batter and mix. Pour in a greased and floured 9x13 pan. Sprinkle the top with the 1/3 cup of grated chocolate. Bake in a preheated 350�F oven for 40 minutes.