Preheat oven to 350°. Spray a 9x13 baking dish with cooking spray. Cut bread into bite-size pieces, about an inch. This a good job for kids. This whole casserole is easy for kids to make.
Transfer bread cubes to the prepared baking dish.
Add eggs, half & half, both sugars, vanilla bean paste, cinnamon, nutmeg, and salt to a large pitcher. Whisk until well combined. This is a perfect job for my beloved stick blender.
Carefully pour the egg mixture over the bread cubes.
Let the bread mixture hang out to soak up the liquid while you make the streusel.
To make the streusel, add flour, brown sugar, cinnamon, nutmeg, salt, and vanilla bean paste to a bowl.
Pour the butter over and combine with a fork. It will make nice clumps of topping.
Top the French toast with the streusel. Try to get it as even as you can. And try not to eat it all before it hits the casserole, it is so good.
Bake for 45 to 50 minutes, or until it has puffed and is golden brown. Let sit for 10 minutes before serving. Top with a dusting of powdered sugar.