Preheat your oven to 350°F. Cut the sourdough bread into 1-inch cubes and spread them evenly in a greased 9x13-inch baking dish.

In a medium bowl, whisk together the eggs, milk, heavy cream, vanilla extract, and sugar until smooth. Pour the custard over the bread cubes, ensuring each piece is soaked.

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

Prepare the topping by mixing the flour, brown sugar, cinnamon, and salt in a bowl. Cut in the cold butter until crumbly. Refrigerate until ready to bake.

Before baking, ensure your oven is preheated to 350°F. Remove the plastic wrap and sprinkle the topping over the bread mixture.

Bake for 45-60 minutes, or until the top is golden brown and the custard is set.