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 the baking dish.
In a medium mixing bowl, whisk together the eggs, whole milk, vanilla extract, salt, and ground cinnamon. Pour the custard mixture evenly over the arranged bread slices.
Cover the baking dish tightly with plastic wrap and refrigerate overnight (or at least for 4 hours).
Preheat your oven to 350°F (175°C). Remove the baking dish from the refrigerator and let it sit at room temperature for about 15 minutes. Bake uncovered for about 35-40 minutes until golden on top and set in the middle.