In small bowl, combine butter and cocoa; stir in sour cream. In large bowl, stir together flour, sugar, baking soda and salt. Add cocoa mixture and eggs; stir until dry ingredients are moistened. Stir in coconut and nuts. Spread batter into prepared pan.
Bake 30 to 35 minutes or until wooden pick inserted in center comes out clean. Cool completely in pan on wire rack. Sift powdered sugar over top, if desired. 9 to 12 servings.