Stir together first 3 ingredients until blended; cool.
Combine flour and next 3 ingredients in a large mixing bowl. Add cocoa mixture, egg yolks, oil, and vanilla; beat at medium speed with an electric mixer until smooth.
Beat egg whites and cream of tartar at high speed with an electric mixer until foamy. Fold into batter. Pour batter into 4 greased and floured 9-inch round cakepans.
Bake, 2 layers at a time, at 325° for 17 to 20 minutes or until a wooden pick inserted in center comes out clean. Cool layers in pans on wire racks 10 minutes. Remove from pans, and cool on wire racks covered with plastic wrap or wax paper to prevent layers from adhering to racks.
Spread about 1/2 cup Chocolate-Coffee Buttercream Filling between layers. Frost with Chocolate Fudge Frosting. Garnish, if desired. Store in refrigerator.