Preheat the oven to 350°F. Butter or spray a medium casserole or baking dish. Set aside.
In a medium bowl, combine ¾ cup brown sugar, ¼ cup flour, ¾ cup chopped pecans and pinch of sea salt. Add ¼ cup unsalted butter (melted) and toss until a crumbly mixture combined. Set aside.
In another medium bowl, beat together 2 cups cooked, mashed sweet potatoes, ¾ cup sugar, ¼ teaspoon sea salt, ½ teaspoon vanilla extract, ¼ cup butter (softened) and 1 egg until smooth with a hand mixer (or by hand).
Pour the sweet potato mixture into the prepared dish. Sprinkle evenly with crumble topping.
Bake until the edges are bubbly and slightly puffed and the topping is browned and fragrant.
Remove from oven and cool for 20-30 minutes before serving.