Coat an 11-by-7-inch or other 2-quart baking dish with cooking spray. Place the bread cubes in the dish. In a blender or a medium bowl with an electric mixer, beat the eggs, egg whites, milk, vanilla extract, salt, and 1 tablespoon of the sugar until blended. Pour the mixture over the bread, pressing until saturated. Let the cubes stand for 15 minutes, or cover and refrigerate them overnight, to soak up the liquid.
Heat the oven to 350°F. If the baking dish has been chilled, remove it from the refrigerator 20 minutes before baking.
Meanwhile, melt the butter in a medium skillet over medium heat. Stir in the bread crumbs, cinnamon, and remaining 3 tablespoons of sugar; cook until the crumbs are saturated and lightly toasted, about 2 minutes. Scatter them over the top of the casserole.
Bake until a knife inserted near the center comes out clean, 35 to 40 minutes. Let the casserole cool slightly, cut it into squares, and serve.
Tip: To make Apple French Toast Casserole, lay two peeled and sliced apples over the bottom of the prepared dish. Combine 1 tablespoon of sugar, ¼ teaspoon of cinnamon, and ½ teaspoon of flour and scatter the mixture over the apples. Top with the bread cubes and continue with the recipe.