Heat oven to 350°F. Line 2-1/2-inch muffins cups with paper or foil bake cups.
Place butter in large microwave-safe bow. Microwave at HIGH (100%) 1-1/2 minutes or until melted. Add cocoa; stir until smooth. Add brown sugar and granulated sugar; stir with spoon until well blended. Add eggs and vanilla; beat well with spoon. Add flour and 1 cup nuts; stir until well blended.
Fill prepared muffin cups about 2/3 full with batter. Sprinkle about 1 teaspoon remaining nuts over top of each.
Bake 22 to 25 minutes or until tops are cracked and feel firm (will be moist inside). Cool in cups on wire rack about 15 minutes. Remove from cups. Cool completely.