Mash your banana in a shallow bowl or baking dish. Try to get it as smooth as you can but it's ok to have some lumps. Whisk together the eggs, milk, cinnamon, vanilla, and peanut butter if using.
Soak each slice in the mixture for 10-15 seconds per side.
Heat skillet or griddle and melt butter under medium-low to medium heat. Once it is bubbly, add the soaked bread and cook for 2-3 minutes on each side, until golden brown. If browning too quickly, turn the heat down. Enjoy with your favorite toppings.
Notes
Bread slices - you can use any kind! Brioche bread, challah, French bread, Texas toast, sandwich bread
Blend all the ingredients using a food processor or blender if you prefer a completely smooth batter.
Storage: refrigerate for 2 days or freeze for up to 2 months.