Grease a baking dish and arrange the croissant slices in layers.
In a mixing bowl, whisk together the eggs, milk, cream, sugar, vanilla extract, cinnamon, and salt.
Pour the custard mixture evenly over the croissants, ensuring each piece is well-coated.
In a separate bowl, combine flour, brown sugar, cinnamon, and butter. Use your fingers to create a crumbly texture.
Sprinkle the topping over the croissants in the baking dish.
Cover and refrigerate overnight to allow the croissants to absorb the custard and the flavors to meld.
Preheat the oven to 350°F.
Bake the casserole for 40 to 45 minutes, until the top is golden brown and the center is set.
Let cool for a few minutes before serving.