1. Grease a 9×13-inch baking dish.

2. Make French toast:

• Stir brown sugar and butter together in a saucepan over medium-low heat until butter is melted and sugar has dissolved, 2 to 4 minutes.
• Pour butter-brown sugar mixture into the prepared baking dish. Scatter French bread pieces over top in a 1.5 to 2-inch layer.
• Whisk milk, eggs, and vanilla together in a bowl until well combined. Pour over bread pieces, then press bread down with a spatula so the bread absorbs the liquid. Cover the dish with plastic wrap and refrigerate overnight.

3. When ready to bake, remove French toast from the refrigerator and discard plastic wrap. Preheat the oven to 450⁰F.

4. Make topping:

• Mix brown sugar and cinnamon together in a small bowl. Sprinkle over the French toast.

5. Bake in oven uncovered until casserole is browned and bubbling, about 30 minutes. Cover with aluminum foil if the top is browning too much.

6. To serve, cut into squares and place upside down onto plates.