Preheat oven to 400°. Arrange sweet potatoes on a foil-lined baking sheet and roast for one hour. Remove from oven and let cool. Once cooled, peel the potatoes and set aside.
Reduce oven temp to 375°. Grease a medium-size casserole dish with butter.
For the crust
Combine brown sugar, flour, nuts and butter in mixing bowl. Chill in the fridge until ready to use. This helps the crumble maintain its form and not melt into the sweet potatoes.
For the mix
Combine sweet potatoes, sugar, salt, vanilla, eggs and butter in the bowl of a stand mixer fitted with the paddle attachment in the order listed. Beat thoroughly for 3-4 minutes until light and slightly fluffy. If the mix is too dense, ass a splash of milk and continue mixing.
Pour mixture into the baking dish. Bake for 25 minutes.
Sprinkle the surface of the sweet potato mixture evenly with the crust mixture and return to oven for 15 minutes or until the crust is golden brown. Allow to set at least 30 minutes before serving.