Beat the eggs, milk and salt together in a shallow dish wide enough to lay a slice of bread flat. Add the cinnamon now if you are using it.
Lay a slice of bread in the dish and let it soak for about 20 seconds, then flip and soak the other side. Day-old bread drinks up the egg without falling apart, so go gently with fresh bread.
Melt the butter in a frying pan over a medium heat until foaming but not browning.
Fry two slices at a time for about 2 minutes per side until deep golden, turning the heat down if the butter browns too fast. Keep them warm on a plate while you cook the rest.
Dust with the sugar and eat while hot, with the middle still a little custardy.