Cut your bread into one-inch cubes and place them evenly in a greased 9x13 inch baking dish.

In a large mixing bowl, whisk together the eggs, heavy cream, milk, vanilla extract, cinnamon, and 1/2 cup of brown sugar until well combined. Pour this mixture over the bread cubes, ensuring all pieces are soaked.

Gently toss the bread to make sure it's fully coated with the liquid. Cover the dish with plastic wrap and refrigerate it overnight.

For the topping, combine the remaining brown sugar with granulated sugar in a small bowl. Cut in the cold butter until the mixture resembles coarse crumbs. Keep this in the fridge until you're ready to use it.

Preheat your oven to 350°F. Take the casserole out of the fridge, remove the plastic wrap, and evenly sprinkle the sugar topping over the casserole.

Cover the dish with aluminum foil and bake for 40 minutes. Then, uncover and bake for an additional 5-10 minutes, or until the top is golden brown. Serve warm and enjoy!