Heat oven to 350ºF. Prepare cake batter and bake in 2 (9-inch) round pans as directed on the package. Cool cakes in pans for 10 min. Invert onto wire racks; gently remove pans.
Microwave the chocolate and butter in a small microwaveable bowl on HIGH for 2 minutes or until the butter is melted. Stir until the chocolate is completely melted.