Prepare Classic American Spongecake.
Prepare Chocolate-Butter Glaze, and store in a covered container in the refrigerator if not using right away.
Prepare Chocolate Flakes. Store in a covered container in the freezer until ready to use.
Pour the half-and-half into a
Split the cake evenly into three layers, each about
Lift the top two layers, one at a time, and set aside (this is easy because each layer is spongy and thick).
Stir the vanilla and rum into the cold custard. Whip the heavy cream in a 1½-quart mixing bowl to the Chantilly stage, or until it clings softly to the beaters. It should be thick enough to create swirls in the bowl but liquid enough to move if the bowl is tilted. Fold the cream into the custard mixture.
Place the bottom cake layer in the bowl and cover it with half the filling. Spread evenly with a rubber spatula. Center the middle layer on top, and cover it with the remaining filling. Center the last layer on top. Cover the dessert with plastic wrap and refrigerate.
Spread the glaze over the top of the dessert evenly with a metal or rubber spatula. Sprinkle Chocolate Flakes around the rim of the dessert.
Refrigerate for up to 1 to 2 hours before serving.