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 cinnamon. Pour the custard evenly over the arranged bread slices, pressing down gently on each slice.
Cover the baking dish with plastic wrap and refrigerate overnight (or at least for 4 hours).
Preheat your oven to 350°F (175°C). Let the dish sit at room temperature for about 15 minutes before baking. Bake uncovered for about 35-40 minutes until golden brown on top.
Allow the casserole to cool slightly before inverting each slice so that its caramelized side faces up. Serve warm with fresh berries or dusted with powdered sugar.