1. Grease a 9x13 inch baking dish.
2. Place the bread cubes in the baking dish.
3. In a mixing bowl, whisk together eggs, milk, heavy cream, sugar, brown sugar, and vanilla extract.
4. Pour the egg mixture over the bread cubes, making sure all the bread is soaked.
5. Cover the dish with plastic wrap and refrigerate for at least 2 hours or overnight.
6. Preheat the oven to 350°F (175°C).
7. In a small bowl, mix together flour, brown sugar, cinnamon, and salt. Add the cold butter pieces and mix until the mixture resembles coarse crumbs.
8. Sprinkle the topping mixture evenly over the soaked bread cubes.
9. Bake in the preheated oven for 45 minutes, or until the top is golden brown and the casserole is set.
10. Let cool for a few minutes before serving. Enjoy!