Soften butter. Beat at medium speed with electric mixer about 2 minutes or until soft and creamy. Gradually add sugars, beating at medium speed 5 to 7 minutes.

Add egg whites, one at a time, beating until they disappear.

Combine flour, baking soda, and cocoa. Add to creamed mixture alternating with sour cream, beginning and ending with flour mixture. Mix at lowest speed just until blended after each addition.

Stir in vanilla.

Spoon into a greased and floured 10-inch tube pan. Bake at 325 degrees for 1 hour and 20 minutes or until wooden pick inserted in center comes out clean.

Cool pan on wire rack for 10 minutes; remove from pan and cool completely on a wire rack.