Make the Streusel: Combine all streusel ingredients in a bowl, mixing well until resembles chunky, damp sand. Cover and keep in fridge to chill.Grease a 9x13 baking dish with butter on bottom and up sides. Place 12 cups of bread cubes in dish. In a large bowl, combine eggs, milk, sugar, salt, and vanilla, whisking to thoroughly incorporate. Pour egg mixture evenly and slowly over the bread cubes. Gently toss to ensure even coating of all bread cubes. If there seems to be too much excess liquid, add another cup of bread cubes until liquid seems mostly soaked up (there will be a thin layer of excess liquid at the bottom, which is ok.)Cover and place in fridge to chill overnight, if making ahead of time (gently toss soaked bread cubes again before baking.)Preheat oven to 350F. Sprinkle chilled streusel evenly over the top of casserole.Bake for 45-60 minutes, or until topping is nicely browned and bubbly. If it starts looking too browned at the 45 minute mark, loosely tent with foil and continue baking as needed.Allow to cool about 10-15 minutes, and serve warm with maple syrup.