In a small saucepan over medium heat, melt the unsalted butter, brown sugar, and corn syrup together. Stir occasionally until the mixture bubbles and thickens, about 3-4 minutes. Pour the caramel sauce into the bottom of a 9×13-inch baking dish.
Slice the loaf of French bread into 1-inch slices and arrange them in a single layer over the caramel base in your baking dish.
In a medium mixing bowl, whisk together the eggs, whole milk, vanilla extract, salt, and optional ground cinnamon. Pour the custard mixture over the arranged bread slices.
Cover the baking dish with plastic wrap and refrigerate overnight or for at least 4 hours.
Preheat your oven to 350°F (175°C). Remove the baking dish from the fridge and let it sit at room temperature for about 15 minutes. Bake uncovered for 35-40 minutes until golden brown.
Allow the casserole to cool for a few minutes before serving. Invert each slice so that the caramelized side is facing up.