At least 1 day before decorating cake, make two color shades of royal icing violets using tip 101 on waxed paper square attached with icing to Flower Nail #7; add tip 3 yellow dot flower centers. (Make extras in case of breakage.) Allow to dry completely.
Spray three 9-inch round baking pans with vegetable pan spray. Prepare Hummingbird Cake batter; divide between the prepared pans (Note: layers will be thin). Bake 30-35 minutes. Cool in pan 10 minutes; remove from pan and cool completely.
Tint buttercream icing light violet. Stack cake layers on prepared cake circle with icing between layers; spatula ice cake smooth. Mark cake sides at 1-in. intervals; pipe tip #48 vertical lines (serrated side up) at marks. Pipe three tip #48 lines crossing each other on the cake top; pipe a line around bottom border. Carefully remove violets from waxed paper; arrange flowers on cake top and top and bottom borders.