Place 6-inch cake, cut side down on tray; spread 1/3 cup frosting over cake. Top with remaining cake round, cut side up. Trim a small amount of edge off custard cupcake so it can stand on top of 6-inch cake round. Insert wooden skewer through top of head through cake rounds for stability; trim skewer as needed. Spread thin layer of frosting over side and top of layered cake and bunny head to seal crumbs. Use frosting to adhere cupcakes to front of 6-inch round cakes for feet. Use toothpicks to secure feet, if necessary; spread thin layer of frosting over feet to seal crumbs. Cut mini cupcake in half; place cut side down on top of cake for bunny arms. Cut 3 marshmallows in half lengthwise. Place 3 marshmallow halves, cut sides down, on top of each cupcake to make toes of feet. Place 1 full marshmallow behind cake to make bunny tail. Freeze cake 30 to 45 minutes to set frosting.