2 ripe but firm bananas cut into large cubes (optional)
2 tbsp butter
Preheat oven to 350°F. Lightly spray sides and bottom of 13x9x2-inch glass baking dish with cooking spray.
Heat milk, cream and sugar in a heavy saucepan over medium-high heat. Stir until sugar dissolves and tiny bubbles form around edge. Mixture should be almost, but not quite, at a boil. Remove from heat.
Mix eggs, egg whites and vanilla in a large bowl to blend. Gradually add hot milk mixture to the egg mixture, stirring with a whisk to prevent eggs from cooking. Add bread cubes and Grape-Nuts to warm milk/egg mixture and let stand until bread and cereal absorb some of custard, stirring occasionally (about 30 minutes).
Heat sauté pan over medium-high heat and add butter. Once melted, add the diced bananas and sauté for 3 minutes or until lightly browned. Add bananas to bread/cereal mixture, stirring gently to combine. Transfer to prepped baking dish. Cover with foil and bake at 350° for 50 minutes or until browned and set.