Grate the cocoa bar. Boil and strain to make about 1 cup of unsweetened cocoa “tea”. Note: About 3 tbsp should do.
Cream together butter, shortening and sugar in large mixing bowl.
Add eggs, vanilla and cocoa tea liquid; blend well.
Combine dry ingredients except the cocoa powder. Alternately add dry ingredients and cocoa powder to creamed mixture.
Pour into 2 greased, floured 9-inch round layer pans or 2 8-inch square pans. Bake in preheated 350 degrees F oven 35 to 40 minutes for 9-inch layer; or 40 to 46 minutes for 8-inch square layer or until cake tester comes out clean. Cool 10 minutes; remove from pans. Cool completely; frost as desired.