In large bowl, beat 1/2 cup softened butter with electric mixer on medium speed 30 seconds. Gradually add granulated sugar, beating well. Beat 2 more minutes. Beat in vanilla and egg until blended. On low speed, alternately add flour mixture and cocoa mixture, beating just until blended. Divide batter among muffin cups. Bake 20 to 25 minutes or until toothpick inserted in center comes out clean. Cool 5 minutes. Remove from pan to cooling racks. Cool completely, about 30 minutes.