To make it...
Whisk the milk, eggs, and sugar in a medium saucepan. Add the butter and chocolate and heat over low heat, stirring only until the butter and chocolate melt. Stir in the vanilla. Lightly coat a 13-by-9-inch baking dish with cooking spray. Arrange the bread slices in 3 layers. Pour the chocolate sauce over the bread. Cover with plastic wrap and place another baking dish on top to weigh down the bread. Set aside for 1 hour to allow the bread to absorb the sauce. (The pudding can be made to this point up to 1 day ahead. Refrigerate.)
Heat oven to 325º F. Remove the top baking dish and plastic. Bake uncovered 35 to 40 minutes or until set. Serve warm with whipped cream.
To fake it...and save 1 hour, 10 minutes
Thaw two 13-ounce packages frozen French toast and cut into 1 1/2 in cubes. Heat oven to 325º F. Lightly coat a 13-by 9-inch baking dish with cooking spray. Place the toast in the dish. Beat 4 cups milk, one 12-ounce can chocolate syrup, and 2 eggs; pour over the toast. Cover with foil and bake 10 minutes. Remove foil and stir. Bake, uncovered, 20 to 25 minutes longer or until set. Serve warm with canned whipped cream.