Several days before decorating cake, make roses. Prepare 6 cups royal icing; tint 3 cups icing dark mauve and the remaining 3 cups light mauve. Using the flower nail, make 20 tip 104 roses with tip 12 bases in each shade. Let dry completely. Make extras to allow for breakage.
Bake and cool 1-layer round cake. Tort layers. Stack cake layers on foil-wrapped cake board using buttercream icing as a filling.
Spatula ice cake smooth with buttercream icing; let set about 15 minutes. Dip pattern press into cornstarch, then randomly imprint design on cake sides. Using buttercream, pipe tip 2 balls on imprinted areas. Add tip 5 bead bottom border. Pipe tip 12 mounds of icing at various heights on cake top; position roses on mounds. Pipe tip 352 leaves with green buttercream icing.