Place butter in large microwave-safe bowl. Microwave at MEDIUM (50%) 2 to 2-1/2 minutes or until melted. Stir in sugar and vanilla. Add eggs, one at a time, beating well with spoon after each addition. Add cocoa; beat until well blended. Add flour, baking powder and salt; beat well. Stir in nuts, if desired. Pour batter into prepared pan.