This is an excellent source of protein, carbohydrates, and (cooked in butter or oil), fat.
Beat the egg, milk, vanilla, and cinnamon together in a flat dish such as a soup plate or pie pan, that will accommodate the bread and make it easy to remove the bread once it's wet. Add 1 slice of bread at a time, turning so that both sides are coated and absorb some of the liquid. Remove the bread, slice by slice, to a plate. Heat butter or oil on a large griddle or skillet, reduce the heat, and brown the bread briefly on each side. Serve with syrup, honey, cooked or raw fruit, or plain.