Trim bottom layers; stack on cake round, with 3/4 cup buttercream spread over top of each. Add top layer; spread 1 cup buttercream over entire cake. Refrigerate 40 minutes until firm. Tint half remaining buttercream pink.
Place cake on rotating stand. Lay flat a 12-inch pastry bag with #103 tip. Fill half of bag from tip to top with pink buttercream, other half with white. Hold bag vertically, slot of tip perpendicular to cake; use swift back-and-forth motion 1 inch wide to make ruffle, pulling tip up side.
Turn cake stand slowly, piping circular ruffle over top.