In a medium saucepan over medium heat, melt 4 tablespoons of unsalted butter, then stir in 1 cup of brown sugar and 1/4 cup of corn syrup. Stir for 3-4 minutes until bubbly and smooth, then pour into a 9x13-inch baking dish.

Slice a day-old loaf of French bread into 1-inch thick pieces and arrange them in a single layer over the caramel base.

In a large bowl, whisk together 6 large eggs, 2 cups of whole milk, 2 teaspoons of vanilla extract, 1/2 teaspoon of salt, and optional 1 teaspoon of cinnamon until smooth. Pour over the arranged bread, pressing down to soak.

Cover the dish tightly with plastic wrap and refrigerate for at least 4 hours or overnight.

Preheat your oven to 350°F (175°C). Remove casserole from the refrigerator and let sit for 15 minutes. Bake for 35-40 minutes until golden brown.

Let cool for about 5 minutes before serving. Invert slices onto plates to show the caramelized top.