Begin by cutting your chosen sturdy bread, like sourdough, into 1-inch cubes. Scatter these cubes evenly in a greased 9x13-inch baking dish.

In a medium mixing bowl, whisk together large eggs, milk, heavy cream, vanilla extract, and sugar until well combined. Pour the custard evenly over the bread cubes in the baking dish.

Cover the baking dish tightly with plastic wrap and refrigerate for at least 4 hours, or ideally overnight.

In a separate bowl, mix together all-purpose flour, light brown sugar, ground cinnamon, and salt. Cut cold unsalted butter into the mixture until it resembles coarse crumbs.

Remove the casserole from the refrigerator and preheat your oven to 350°F (175°C). Unwrap the casserole and sprinkle the topping over the soaked bread cubes.

Place the dish in the preheated oven and bake uncovered for 45-60 minutes until golden-brown on top and the center is set.

Once baked, remove the casserole from the oven and let it cool for a few minutes before serving. Cut into squares and serve warm.