Cut (or break apart) your brioche bread into 1-inch cubes and place it into a greased 9x13 baking dish.
In a large bowl, whisk together the eggs, heavy cream, milk, maple syrup, vanilla, cinnamon, and salt until combined. Pour this over top of the bread evenly and gently toss, making sure all pieces of bread are able to soak some of the liquid up.
Cover with foil and place in the fridge overnight (or for 4-5 hours).
Make the Streusel
You can make the crumble topping the day before as well by mixing together the cold butter, brown sugar, flour, cinnamon, and salt with a pastry cutter or two forks until you get pea-sized pieces of butter.
Place this in a ziploc baggie or plastic container and leave it in the fridge overnight. You could also make the topping in the morning if needed.
Bake
Preheat your oven to 350° F. Crumble the streusel evenly over the top of the casserole and place in the oven for about 45-55 minutes, until the casserole is baked through in the middle and golden on top. If you notice the casserole browning too quickly, you can add foil on top during the baking process.
Video
Notes
Storing & freezing tips: This can be stored in an airtight container for up to 5 days in the fridge. You can heat it up in the microwave for 30-60 seconds and enjoy it warm!
If you want to cut back on the sweetness, you have two options. Omit the maple syrup from the casserole, or cut in half the topping (or leave it off completely, although I don't recommend this because it's so good)!
If you notice the casserole is starting to brown too quickly while baking, you can take it out, cover it with tin foil and return it to the oven.
Serve with maple syrup- this is great without it as well though!