Whisk egg, milk, cinnamon (if using) and vanilla together.
Dip each slice of bread into mixture and let it soak until the mixture is absorbed. The heartier the bread, the longer this takes. 30 seconds or so on each side.
Heat non stick pan over medium/low heat then add 1 Tablespoon of butter.
Fry soaked bread until golden on both sides. Place in 200 degree oven to keep warm.