Using a 1 1/2-inch cookie cutter, cut out circles from cake. Brush off any crumbs with a pastry brush.
Fit a pastry bag with a plastic coupler, and top cake rounds with icing in a ghost or hat shape,about 1 inch high.
Place a wire rack over a rimmed baking sheet; set aside. Set an iced cake round on a fork; hold over bowl of glaze. Using a large spoon, drizzle glaze evenly over cake and icing until completely covered. Transfer to wire rack. Repeat with remaining rounds. Let set, about 5 minutes.
Place chocolate in a heatproof bowl set over a pan of barely simmering water; stir chocolate until melted. Transfer to a disposable pastry bag or resealable plastic bag; snip off tip with scissors. Pipe eyes and a mouth onto ghosts.
For Witch Hats, make petits fours through step two. Then create a brim using a dab of icing: Affix each petit four to a 2-to-2 1/2-inch chocolate wafer cookie. Proceed to step three, coating with Chocolate Glaze instead of Butter Glaze. In lieu of step four, wrap one or two pieces of licorice lace around the base of the hat immediately after glazing.