Preheat your oven to 350°F (175°C). Grease a 9×13 inch baking dish with 2 tablespoons of unsalted butter.

In a large mixing bowl, whisk together 4 large eggs, 1 cup of whole milk, 2 teaspoons of ground cinnamon, 1 teaspoon of pure vanilla extract, and ¼ cup of maple syrup.

Place half of the 6 slices of thick sliced brioche or challah bread into the greased baking dish. Pour half of the custard mixture over the bread slices.

Add the remaining bread slices on top and pour the rest of the custard mixture over, pressing down gently.

Cover the dish tightly with plastic wrap and refrigerate overnight for at least 8 hours.

Remove plastic wrap and bake for about 45 minutes until golden-brown and puffed.