Place the bread cubes in a buttered casserole dish.
In large bowl, whisk together eggs, milk, half and half, brown sugar, cinnamon, and vanilla extract until combined. Pour evenly over bread cubes. Cover and refrigerate over night.
For the topping:
With a fork or pastry blender, combine brown sugar, flour, cinnamon, salt, and butter until it resembles coarse sand. Cover and refrigerate until ready for use.
Preheat oven to 350 degrees F. Remove French toast from refrigerator and sprinkle crumb topping evenly on top. Bake uncovered for 45-60 minutes until puffed, golden brown, and set in the middle. Baking time will vary depending on size of dish and how well done you prefer the middle. I baked mine closer to an hour. Serve with Golden Barrel Pancake and Waffle Syrup. Refrigerate leftovers.