Cut one banana into thin slices. Line the bottom of seven 6 ounce ramekins with a layer of sliced bananas. Set ramekins aside. Mash one banana well and set aside.
In a medium saucepan, whisk together sugar, cornstarch, salt, nutmeg, and cinnamon. Whisk in about 1/3 cup of the milk until cornstarch dissolves. Gradually pour in the rest of the milk and the butter.
Turn on heat to medium and begin to cook. Use the rubber spatula to slowly stir the pudding constantly in large sweeping motions, scraping the sides and bottom of the pan. When mixture is thickened and coats the spatula, turn heat to low and begin to stir quickly in little circles all over the pan. Simmer for one minute, stirring constantly.
Remove from heat and gently stir in vanilla and mashed banana. Immediately divide the pudding among the six ramekins. Place plastic wrap directly on the surface of the pudding cups and refrigerate for at least two hours. When ready to serve, slice remaining banana and garnish pudding tops with banana slices.