Whisk together eggs, milk, salt, sugar, vanilla, and cinnamon in a flat-bottomed pie plate or baking dish. Place bread slices, one or two at a time, into the egg mixture and flip to make sure both sides of bread are well-coated.
Melt butter in a large skillet or on a griddle. Place bread slices in skillet or on griddle and cook on medium heat until golden brown on each side, about 2-3 minutes.
Serve immediately or keep warm in oven until ready to serve, but no longer than about 30 minutes.
You are simply dipping your bread slices into the egg mixture. If you leave your bread in the mixture for an extended amount of time, it will absorb more liquid and take longer to cook through.