Dust cookie pan cavities and skull candy mold with cornstarch. Press desired fondant color into prepared pan and candy mold. Carefully remove from molds. Allow some shapes to dry flat on cornstarch-dusted cake boards; cover remaining with plastic wrap to keep moist and pliable.
Brush details of rat candy mold with melted candy; refrigerate 5 minutes. Fill mold with additional contrasting melted candy; refrigerate until set, about 20 minutes. Remove from mold.
Bake and cool a 2-layer paisley-shaped cake. Stack cooled cake layers on a foil-wrapped cake board cut to size, using buttercream icing as a filling. Prepare cake for fondant by lightly icing with buttercream.
pieces to cake and around bottom border with fondant adhesive. Top with candy rat.