Grease a 9x13 inch baking dish with butter or non-stick spray.
Place the brioche cubes into the prepared baking dish in an even layer.
In a large mixing bowl, whisk together the eggs, milk, heavy cream, granulated sugar, vanilla extract, cinnamon, nutmeg, and salt until well combined.
Pour the egg mixture evenly over the bread cubes, ensuring all pieces are moistened.
Cover the dish with plastic wrap and refrigerate for at least 8 hours or overnight.
When ready to bake, preheat the oven to 350°F (175°C).
In a small bowl, combine the cold butter cubes, brown sugar, flour, and cinnamon using a pastry cutter or fork until a crumbly texture forms.
Uncover the casserole and sprinkle the streusel topping evenly over the soaked bread.
Bake for 45 to 50 minutes, or until the top is golden brown and the custard is set.
Let rest for 5 minutes before serving.