Whisk the eggs, milk, optional sugar, vanilla, cinnamon, and salt in a shallow dish until combined.
Dip each slice of bread in the custard for a few seconds per side. Let excess drip off so the bread is coated but not soggy.
Melt butter in a skillet over medium heat. Cook the slices in batches for about 2 to 3 minutes per side, until golden and set in the center.
Add more butter as needed and keep cooked slices warm on a rack in a low oven while finishing the batch.
Serve warm with maple syrup, berries, or powdered sugar.