Add apple slices, brown sugar, and butter in a medium saucepan. Cook over medium-high heat until brown sugar has dissolved and butter has melted. Add half the vanilla and half the 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 egg, salt, remaining vanilla, milk, and remaining 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 excess egg mixture to drip off.

Cook toast in a large pan over medium heat 1-2 minutes until lightly browned, then flip and cook the other side 1-2 minutes. Repeat with remaining bread slices. Generously top toasts with apple-cinnamon mixture. Sprinkle with powdered sugar if desired.

Place warm French toast on a plate, then top with the apple cinnamon mixture and dust with powdered sugar. Enjoy with a cold glass of milk!