Beat butter at medium speed with an electric mixer about 2 minutes or until soft and creamy.

Gradually add sugars, beating at medium speed 5 to 7 minutes.

Add eggs, one at a time, beating until yellow disappears.

Combine flour, baking soda, and cocoa; add to creamed mixture, alternately with sour cream and ending with flour mixture.

Mix at lowest speed just until blended after each addition.

Stir in vanilla.

Spoon batter into a greased and floured 10-inch tube pan.

Bake at 325°F for 1 hour and 20 minutes or until a wooden pick inserted in center comes out clean.

Cool pan on a wire rack 10 to 15 minutes; remove from pan, and cool completely on a wire rack.

Sprinkle with powdered sugar, if desired.