Cream butter and shortening, gradually add l 1/2 cups sugar, beating until mixture is light and fluffy.
Add egg yolks, beat at medium-high speed of an electric mixer for 5 minutes.
Combine flour, salt, and soda; mix well.
Add to creamed mixture alternately with sour cream, beginning and ending with flour mixture.
Mix just until blended after each addition.
Stir in vanilla, 1 1/3 cups chocolate morsels, and grated chocolate.
Beat egg whites (at room temperature) until foamy.
Gradually add remaining 1/2 cup sugar, 1 tablespoon at a time, beating until stiff peaks form.
Fold egg whites into batter.
Spoon batter into a greased and floured 10-inch tube pan.
Bake at 325° for 1 hour and 20 to 25 minutes.
Cool in pan 10 minutes; invert onto a serving plate.
Combine powdered sugar and 1 tablespoon milk, stirring well.
Add more milk if a thinner consistency is desired.
Spoon glaze over warm cake.
Sprinkle remaining chocolate morsels evenly over glaze.
Cool before serving.