1 day in advance: Make cookies. Prepare and roll out dough. Cut cookies using third smallest star cutter. Bake and cool cookies. Outline cookie with tip 7 and full-strength royal icing; let set. Flow in with thinned royal icing; let dry overnight. Attach icing decorations with full-strength royal icing.
Bake and cool cupcakes. Ice smooth with buttercream. Position cookie on cupcake, supporting with dots of buttercream as needed.