Add apple slices, brown sugar, and butter in a medium sauce pan. Cook over medium high heat until brown sugar has dissolved and butter has melted. Add 1/4 teaspoon vanilla and 1/2 teaspoon cinnamon and reduce heat to simmer. Cook 6-8 minutes longer. Remove from heat and allow to cool slightly and thicken while you prepare toast.
In a large, wide bowl whisk together eggs, vanilla, milk, and cinnamon. Stir in flour and whisk until smooth. Completely submerge one slice of bread in the mixture, careful to coat both sides. Lift the bread out of the liquid and allow to drip off excess.
Cook toast in a large pan over medium heat 1-2 minutes until lightly browned, then flip and cook other side 1-2 minutes. Repeat with remaining bread slices. Generously top toasts with apple-cinnamon mixture. Sprinkle with powdered sugar if desired.
If you want to save time, you can totally make the apple cinnamon topping the night before. Simply reheat in a saucepan while you grill the bread (or the microwave will work too!).
I don't love leftover French toast. It tends to get too soggy for my taste. If you want leftovers, save the batter in a separate container and make again the next day.
For best results, use thicker cuts of bread (at least an inch) to make this the best French toast you've ever made!