If you have milk chocolate or white baking chips on hand, you can use them as part of the 1/2 cup chocolate chips stirred into the reserved cake mixture.
Serve cake with an easy fudge sauce. In small microwavable bowl, microwave 1/3 cup sweetened condensed milk and 1/2 cup semisweet chocolate chips uncovered on Medium (50%) about 1 minute or until chocolate can be stirred smooth. Stir in 2 tablespoons whipping cream.
Looking for a way to use leftover sweetened condensed milk? Try Easy Caramel Apple Dip: In 1-quart saucepan, mix 1/3 cup sweetened condensed milk, 2 tablespoons butter and 2 tablespoons brown sugar. Heat over medium-low heat until hot and bubbly, stirring constantly; continue to cook 2 minutes. Remove from heat; stir in 1 tablespoon milk or apple juice. Serve warm with apple slices.