In a large bowl, whisk together large eggs, egg yolks, whole milk (or heavy cream), vanilla extract, and cinnamon. Pour this egg mixture into a shallow dish or shallow bowl.

Place the thick slices of bread into the custard mixture. Let each piece of bread soak, turning to coat both sides of the bread. About 30 seconds on each side. A shallow dish makes it easier to dip.

Heat 1 tablespoon of butter in a large skillet over medium heat (or medium-high heat if using thicker bread). Add soaked bread slices and cook until the sides of the bread are golden brown. About 3–4 minutes per side. Place finished slices on a baking sheet in a warm oven to keep them hot while you make the sauce. Continue frying each piece of bread adding a bit more butter as needed for frying.

In the same skillet, melt 4 tablespoons of butter over medium heat. Stir in the dark brown sugar with a wooden spoon until it dissolves. Add cream and cinnamon, then stir until smooth. Add 2 of the sliced bananas and cook for 1–2 minutes, until they soften and become warm bananas. Carefully add the dark rum (and flambé if desired). Cook another couple of minutes until you have a rich banana sauce.

Place French toast slices on a plate and spoon the foster topping over each piece. Top with slices of fresh banana and whipped cream if desired.