Cut a piece of acetate to fit a baking sheet, and tape it onto back of sheet. Coat surface of wood-graining tool with a thick layer of bittersweet chocolate. Drag tool over acetate, rocking back and forth slightly, to create a wood-grained pattern. (If you are not satisfied, roll over each strip again immediately.) Freeze until set, about 3 minutes.

Using an offset spatula, gently spread a layer of white chocolate over the bittersweet chocolate (do not press too hard or chocolate will smear). Freeze until almost set, 1 minute.

Using cutter, cut circles in chocolate, leaving circles in place. Freeze for 10 minutes. Remove tape from 1 side of sheet, and carefully lift acetate. Press out rounds. Frost cupcakes with chocolate frosting. Place 1 round on top of each.