1. Spread cake in a single layer in a 9- by 13-inch pan. Bake in a 450° oven until lightly toasted, 5 to 7 minutes. Place in a small bowl.
2. Meanwhile, in a 1- to 2-quart pan or fondue pan over high heat, boil wine until reduced to 1/4 cup, 6 to 8 minutes.
3. Reduce heat to medium-high, add mascarpone, and stir until boiling, about 4 minutes. Set pan over a candle or on an electric warming tray.
4. Use forks or thin wood skewers to spear cake and apple slices and dunk them into fondue.