For the Spiced Sugar:

1. Whisk together the sugar and spices until well combined. Use half on the bread before allowing the casserole to “soak” overnight and reserve the other half for sprinkling on just before baking.

For the Casserole:

1. At least 8 hours before you want to serve (I often make this the night before so it can set in the fridge overnight), brush some of the melted butter in a large baking dish. Place half of the spiced sugar mixture in a shallow bowl or plate.
2. Brush the remaining butter on each side of each challah slice and then dip in the spiced sugar, turning to evenly coat both sides of the bread. Place the coated bread slices in the buttered pan, arranging them in as even a layer as you can.
3. Whisk together the eggs, milk, vanilla, and salt in a bowl. Pour the mixture over the bread in the pan, pressing the bread gently down to coat them. Wrap the baking dish with plastic wrap and place in the fridge for at least 8 hours.
4. After 8 hours (or the next morning), remove the baking dish from the fridge and let it sit at room temperature for 30 minutes. Preheat the oven to 350°F during this time.
5. Remove the plastic wrap from the baking dish and liberally sprinkle the remaining half of the spiced sugar over top of everything. This will ensure a crispy sugar crust after baking.
6. Bake for 35-40 minutes, or until the custard is set and the bread is golden and crisp.
7. Allow to cool slightly in the pan before serving, drizzled with the Mexican chocolate sauce.

For the Chocolate Drizzle:

1. While the casserole is baking, place the cream in a small saucepan and bring to a simmer. Meanwhile, chop the chocolate and place it in a heat-safe bowl.
2. Pour the hot cream over the chocolate, let it sit for a few seconds, and then whisk until smooth and melted. Whisk in the vanilla.
3. Serve the French toast warm with the chocolate drizzled over top. Serve additional chocolate sauce on the side.