Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking dish.
Place the challah bread cubes in the prepared baking dish.
In a large bowl, whisk together the eggs, whole milk, heavy cream, 1 cup of granulated sugar, vanilla extract, cinnamon, nutmeg, and salt until well combined.
Pour the egg mixture over the bread cubes, pressing down slightly to ensure the bread is well soaked.
In a small bowl, mix the remaining 0.5 cup of granulated sugar, melted butter, and brown sugar, then sprinkle evenly over the soaked bread.
Bake in the preheated oven for 45 minutes or until the top is browned and a toothpick inserted in the center comes out clean.
Notes
For a more decadent version, try adding chocolate chips or raisins before baking. Serve with maple syrup if desired.