In large bowl, beat 1 cup butter with electric mixer on medium speed until creamy. Gradually add brown sugar and granulated sugar, beating well. Add eggs, one at a time, beating after each addition. Add melted chocolate and 1 teaspoon vanilla, beating well. Add flour mixture alternately with buttermilk, beating on low speed after each addition until blended. Divide batter evenly among muffin cups, filling three-fourths full.