Preheat your oven to 350°F (175°C).

Cut your sourdough bread into 1-inch cubes and scatter them in a greased 9x13-inch baking dish.

In a large bowl, whisk together the eggs, milk, heavy cream, vanilla extract, and sugar until well combined. Pour over the bread cubes.

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

Combine the flour, packed light brown sugar, cinnamon, and salt in a bowl. Cut in the cold unsalted butter until crumbly. Refrigerate overnight.

Sprinkle the topping evenly over the soaked bread and bake uncovered for 45-60 minutes until golden brown.

Let the casserole cool for a few minutes, slice, and serve warm.