In advance: Make wheels. Bake and cool 1-layer cake using firm-textured batter such as pound cake. Cut wheels using Circle Cutter. Pour or pipe on melted candy to cover (see technique link). Add 2nd layer if necessary for smooth finish. Let set on waxed paper-covered boards.
Tint dark cocoa candy black. Add a drop or 2 of water to thicken candy for piping. Use cut parchment bag to outline and fill in zigzag tire treads, 1/8 in. wide, 1/8 in. apart. Chill until firm. Attach topper with melted candy.