Combine cocoa, 1/2 cup sugar and hot coffee; let cool.
Cream butter and 1 1/2 cups sugar until light and fluffy.
Add cocoa mixture, sour cream and vanilla.
Add dry ingredients which have been sifted together; mix until smooth.
Beat egg whites with 3/4 cup sugar until stiff.
Fold carefully into batter.
Pour into 3 well-greased 9" layer cake pans.
Bake 30 minutes at 350°.
Cool before removing from pans.