Preheat your oven to 350°F and grease a muffin tin or line with parchment liners.
Cube enough bread to make about 10 cups. (approximately 12 slices of bread)
In a large bowl, place your cubed bread. For best texture, use bread that’s a day or two old or lightly toasted. This helps it soak up the egg mixture without becoming soggy.
In a medium bowl, whisk together the eggs, warm milk, melted butter, vanilla, salt, cinnamon, and sugar. Warming the milk prevents the butter from hardening and helps everything combine smoothly.
Pour the egg mixture over the bread cubes. Toss with a wooden spoon or your hands until everything is coated. Let it sit for about 3 minutes so the custard can soak in, then toss gently again.
Scoop the mixture into your prepared muffin cups, pressing down lightly. You want each one to be full and slightly mounded.
Sprinkle the tops with cinnamon sugar. A sugar duster or fine mesh strainer makes this easier.
Bake for 25–28 minutes or until the tops are golden brown and the centers are set. The cooking time can vary from oven to oven, so be sure to check on it so that the tops don't burn.
Let the french toast cups cool in the pan for a few minutes, then transfer to a wire rack or serve warm.