Lightly grease a 9x13-inch baking dish with cooking spray. Then, cut your bread into 1-inch pieces and put them in the baking dish.
In a medium mixing bowl, whisk the eggs. Then add and whisk the heavy whipping cream and milk. Add the brown sugar, cinnamon, salt, and vanilla, and whisk again. Pour the egg mixture over the bread cubes. Gently toss the bread in the baking dish to ensure all pieces are covered in the egg mixture. Cover the casserole and place it in the refrigerator overnight or until ready to bake. Allow 1 hour before baking for the bread to absorb the egg mixture.
Prepare the French toast casserole topping. Mix the brown sugar, sugar, and cinnamon in a small bowl. Then, cut the cold butter in the cinnamon sugar until the butter and sugar form a fine crumble using your hands, a fork, or a pastry blender. Place the topping in a covered dish in the refrigerator until it's time to bake the casserole.
To bake the French toast casserole: First, preheat the oven to 350ºF. Uncover the French toast casserole, sprinkle the topping evenly over the top, and bake the casserole for 45-50 minutes or until the top is golden and the casserole is set.
Allow the french toast to cool for at least 10 minutes before serving. Top with homemade syrup, fresh berries, homemade blueberry compote, or any other desired toppings. Enjoy!