In a shallow dish or pie plate, whisk together the eggs, milk, sugar, cinnamon, vanilla extract (if using), and salt until well combined and slightly frothy.
Place the bread slices into the egg mixture, allowing each side to soak for about 30 seconds to 1 minute. Don't let them get too soggy, especially if your bread is very soft.
Heat 1 tablespoon of butter or oil in a griddle or large non-stick pan over medium heat. Once the butter is melted and shimmering (or the oil is hot), carefully place the soaked bread slices onto the hot surface. You may need to cook in batches depending on the size of your pan.
Cook for 2-4 minutes per side, until golden brown and cooked through. Flip gently and cook the other side until equally golden and delicious. Add more butter or oil as needed between batches.
Serve immediately.