Lightly grease a 9x13 in baking dish and layer the challah pieces into the dish.

In a large bowl, whisk together the eggs, milk, heavy cream, brown sugar, vanilla, 1 teaspoon cinnamon, nutmeg, and salt. Pour the egg mixture over the bread, cover the baking dish with plastic wrap, and let rest in the fridge for at least 2 hours, or up to 1 day.

When ready to bake, preheat the oven to 375 and remove the dish from the fridge. In a small bowl, stir together the granulated sugar with the remaining 1 teaspoon cinnamon. Drizzle the melted butter evenly over the top of the dish, then sprinkle the cinnamon-sugar on top. Bake until puffed and deep golden brown, 35 to 45 minutes.