1. Lightly butter the bottom and sides of a 9×13-inch baking dish.
2. Cut bread into one-inch chunks and place in the dish.
3. In a large mixing bowl, whisk eggs. Add milk, brown sugar, cinnamon, salt, and vanilla and mix well. Pour evenly over the bread cubes. Gently toss the bread around a little to make sure the top and bottom pieces all get dipped in the egg mixture. Spread out evenly, cover the dish, and refrigerate overnight.
4. For the topping: In a bowl, combine the sugars and cinnamon. Add butter and form a crumble with your hands. Refrigerate until ready to use.
5. Preheat oven to 350 degrees Fahrenheit.
6. Sprinkle crumb topping over the casserole. Bake for 40 to 50 minutes until the casserole is set and golden brown.
7. Cool for five minutes before cutting into squares. Serve with maple syrup, powdered sugar, or fresh fruit.