In advance: Make flowers. Using yellow royal icing and flower nail, pipe a tip 103 rose with tip 10 base for each brownie. Make extras to allow for breakage and let dry.
Bake and cool brownies in oblong pan; remove from pan. Score 2 1/2 in. squares using brownie cutter; cut with knife. Ice smooth with Fudge Icing. Comb wave pattern using small-tooth edge of Decorating Comb. Position rose.