Whisk the eggs, milk, sugar, vanilla, cinnamon and salt together in a wide shallow dish until completely smooth — no streaks of white left.
Heat a large frying pan over a **medium-low** heat and add a knob of the butter.
Lay two slices of bread in the custard and leave for about 30 seconds, pressing gently, then turn and do the same on the other side. Lift out and let the excess drip off.
Cook for 3–4 minutes on the first side, until deep golden. Resist turning the heat up.
Flip and cook another 3–4 minutes, until the second side is golden and the middle feels set rather than squashy when pressed.
Keep warm on a wire rack in a 100°C oven while you cook the rest, adding fresh butter for each batch.
Serve with maple syrup, and berries or crisp bacon if you like.