In a medium saucepan, melt ½ cup unsalted butter, 1 cup brown sugar, and ¼ cup corn syrup over medium heat, stirring continuously for 3-4 minutes until the mixture is bubbling and glossy. Pour the caramel into the bottom of a 9x13-inch baking dish and spread it evenly.

Slice a loaf of French bread into 1-inch thick pieces and arrange them over the caramel layer, overlapping the slices slightly to ensure each piece soaks up the custard.

In a large mixing bowl, whisk together 6 large eggs, 2 cups whole milk, 2 teaspoons vanilla extract, ½ teaspoon salt, and optional ½ teaspoon ground cinnamon until completely smooth. Pour this creamy custard mixture over the bread.

Cover the dish with plastic wrap and refrigerate overnight for optimal absorption.

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

After baking, allow the casserole to cool slightly for about 10 minutes. Carefully invert each slice onto serving plates and serve warm, garnished with fresh berries or powdered sugar if desired.