Whisk together the egg and milk in a medium-sized bowl. Add the vanilla, sugar, and cinnamon and whisk until well incorporated.
Place bread slices, one or two at a time, into the egg mixture and flip to make sure both sides of bread are well-coated.
Melt the butter in a medium-sized skillet or griddle over medium heat. Place one of both slices of the bread into the skillet and cook slices until golden brown, then flip to cook the other side. Serve with syrup.
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Notes
Whisk the Custard Well: Lightly whisk the egg before adding the milk and spices to create a smooth mixture. For extra flavor, try a pinch of nutmeg or a bit of citrus zest.
Let the Bread Soak: Allow the bread to sit in the custard for at least 30 seconds. Thicker slices may need more time to fully absorb the mixture.
Preheat the Skillet: Heat the skillet or griddle over medium while the bread soaks. This ensures even cooking without burning the outside.
Make a Casserole Version: For a baked twist, use croissants or leftover pastries, layer them in a small dish, pour the custard over, and bake for a single serving French toast casserole.