Cut the challah bread into large cubes. Slightly stale bread works best because it absorbs the custard mixture without becoming soggy.

Place the bread cubes evenly in a greased 9x13-inch baking dish.

In a large mixing bowl, whisk together the eggs, whole milk, heavy cream, granulated sugar, brown sugar, vanilla extract, cinnamon, and salt.

Whisk until the mixture is completely smooth and the sugars have dissolved.

Slowly pour the custard mixture over the bread cubes, making sure every piece gets coated.

Press the bread down gently with a spatula so it absorbs the liquid.

Cover the baking dish with plastic wrap and place it in the refrigerator.

Let it chill for at least 8 hours or overnight. This step allows the bread to soak up the custard and develop incredible flavor.

In a bowl, combine brown sugar, flour, cinnamon, and salt. Add the cold butter cubes and mix with your fingers or a pastry cutter until crumbly.

The mixture should resemble coarse crumbs.

Preheat the oven to 350°F (175°C).

Remove the casserole from the refrigerator and let it sit at room temperature for about 20 minutes.

Sprinkle the streusel mixture evenly across the soaked bread.

This topping will melt slightly and create a delicious caramelized crust.

Bake the casserole for 45 minutes until the top is golden brown and the center is set.

The aroma filling your kitchen at this point will be absolutely irresistible.

Let the casserole rest for about 10 minutes before serving.

Top with maple syrup, powdered sugar, or fresh berries.

Enjoy warm.