Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish with butter.
In a large mixing bowl, whisk together the eggs, whole milk, heavy cream, granulated sugar, vanilla extract, ground cinnamon, and salt until well combined.
Add the cubed bread to the egg mixture, gently folding to ensure all pieces are coated. Allow the mixture to sit for about 15 minutes to soak.
Pour the bread mixture into the prepared baking dish, spreading it evenly.
In a small bowl, combine the brown sugar and a pinch of cinnamon. Sprinkle this mixture evenly over the top of the casserole.
Bake in the preheated oven for 45-50 minutes, or until the top is golden brown and the center is set.
Let the casserole cool for about 10 minutes before serving. For a crispy top, you can use a kitchen torch to caramelize the brown sugar topping just before serving.