In advance: Prepare and roll out cookie dough. Cut two cookies using largest heart cutter; bake and cool.
Bake and cool Carriage and Wonder Mold cakes. Cut Wonder Mold cake vertically in half; reserve other half for another use. Trim carriage cake by cutting away top of carriage point, seat area and back. Ice window and wheel well areas smooth.
Position cake on board with wheels at top. Position heart cookies for back legs, angled so that bottom curve appears smaller, and under Wonder Mold half for body. Trace shape on board; cut and wrap with foil. Reposition cookies and cakes. Trim off portion of cookies below Wonder Mold cake.
Add icing to front wheel to equalize size of eyes. Pipe tip 1A mound for eyes (pat smooth with finger dipped in cornstarch). Mark mouth and nostril areas with toothpick; ice areas smooth and outline with tip 3. Outline eyelid with tip 3. Cover head with tip 16 stars. Pipe tip 1A front legs and toes on body; overpipe toes. Cover body and legs with tip 16 stars. Overpipe toes with tip 3 outlines; outline leg creases. Pipe in tip 3 dot pupils (flatten with finger).
The star tip creates the most celebrated, easily accomplished decorations! The serrated edges of the star tip make ridges in the icing as you squeeze it out. Because fill in requires so much piping from the same bag, keep replenishing icing in bag.