Preheat oven to 400 degrees
Use a fork to poke holes in the potatoes and place them on a baking sheet lined with parchment paper. Bake for about 1 hour. They are done when you can easily poke a fork through them.
Use a knife to cut them open and let them cool slightly.
Scoop the potatoes out into a large bowl.
Add the sugar, salt, vanilla extract, eggs and melted butter.
Use a hand mixer and mix until smooth.
Spread into a greased 9x13 baking dish.
Bake for 35-45 minutes or until the center is set.
In a bowl, mix together the ingredients for the streusel topping.
Spread evenly over the casserole.
Bake for 15-20 more minutes, or until the streusel is a golden brown color and it is no longer crumbly.
Let sit for at least 10 minutes before serving.