1Heat oven to 350°F. Line 13x9-inch pan with foil, extending foil over edges of pan. In medium bowl, stir cookie mix, butter and egg until soft dough forms. Press dough into pan evenly with floured fingers.
2Bake 20 to 25 minutes or until edge is light golden brown. Cool 15 minutes in pan on wire rack. Pick up foil edges and carefully remove cookie from pan to cooling rack; cool 15 minutes. Turn cookie upside down onto another cooling rack; remove foil. Place cookie on large serving platter.
3Spread frosting on top and sides of cookie. Place foil baking cup in one corner of cookie; fill with yellow jelly beans. Arrange remaining jelly beans on cookie to look like a rainbow. For 12 bars, cut into 4 rows by 3 rows.