"I've been refining this recipe for 10 years. I've finally got it just right. I hope you like it, too! It's easily expandable, and any extras can be refrigerated or frozen and reheated in the microwave."
In a flat bottomed, square, microwave safe container, put the milk and butter. Microwave on high until the milk is warm to the touch and the butter begins to melt, about 1 minute. Stir well until the butter melts fully. Stir in the sugar, if using. Whisk the egg into the mixture until well blended. Sprinkle the cinnamon on top, and stir gently. Some of the cinnamon will float on top. (see note)
Heat a non-stick skillet on medium high heat. Spray very lightly with cooking spray. Spray very lightly again before each slice is placed into the pan.
Lay one slice of bread on top of the mixture to moisten (do not push down into the batter) and using your fingers and a fork, quickly flip over to coat the other side. Again using your fingers and a fork, quickly place into the heated pan. Cook until it is firm enough to slide a spatula underneath easily, and takes on a "marbled" look, with some dark and light areas, about 1 minute. Flip and cook the other side to the same doneness. Transfer to a plate and cover with a clean dish towel to retain heat until all are cooked. Keep dipping and cooking until the mixture is depleted.