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 coarsly, 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 350oF oven for 40 minutes.