In a small saucepan over medium heat, melt together the unsalted butter, brown sugar, and corn syrup. Stir occasionally as the mixture begins to bubble and thicken—this should take around 3-4 minutes. Pour this mixture evenly into the bottom of a 9×13-inch baking dish.
Slice your loaf of French bread into 1-inch slices and arrange them in a single layer over the caramel base in the baking dish.
In a medium mixing bowl, whisk together the eggs, whole milk, vanilla extract, salt, and cinnamon (if using) until smooth and slightly frothy.
Pour the custard mixture evenly over the arranged bread slices in the baking dish. Press down gently on the bread slices to ensure they soak up the custard. Cover the dish with plastic wrap and refrigerate it overnight or for at least four hours.
Preheat your oven to 350°F (175°C). Let the casserole sit out for about 15 minutes to come closer to room temperature, then bake uncovered for 35-40 minutes until golden brown and the custard has set.
Allow the casserole to cool for a few minutes before serving. Invert each slice so that its caramelized side faces up when serving.