Begin by gathering all your ingredients and greasing a 9x13-inch baking dish to prepare it for the French toast.

In a medium saucepan over medium-low heat, combine the brown sugar and butter. Stir continuously until the butter is fully melted and the sugar is completely dissolved, which should take about 2 to 4 minutes. Once combined, pour the mixture evenly across the bottom of the greased baking dish.

Evenly distribute the pieces of French bread over the butter and brown sugar layer, creating a layer that is about 1 1/2 to 2 inches thick.

In a large bowl, whisk together the milk, eggs, and vanilla extract until the mixture is smooth and uniform. Pour this mixture over the bread pieces, ensuring that all the bread is coated. Gently press the bread down with a spatula so it absorbs the liquid. Cover the dish with plastic wrap and refrigerate it for at least 8 hours, or preferably overnight, to allow the flavors to meld.

When you’re ready to bake, take the dish out of the refrigerator and remove the plastic wrap. Preheat your oven to 450 degrees F (230 degrees C).

To make the topping, combine the brown sugar and cinnamon in a small bowl. Sprinkle this mixture evenly over the soaked bread.

Place the dish in the preheated oven and bake for about 30 minutes, or until the casserole is golden brown and bubbling. If you notice the top browning too quickly, cover it with aluminum foil during baking to prevent over-browning.

Once done, allow the French toast to cool slightly before cutting it into squares. For an elegant presentation, invert the squares onto serving plates, showcasing the caramelized bottom layer on top. Enjoy this rich and flavorful breakfast dish!