Preheat the oven to 350°F. Butter or spray a medium casserole or baking dish. Set aside.
In a medium bowl, combine brown sugar, flour, pecans and sea salt. Add melted butter and toss until a crumbly mixture combined. Set aside.
In another medium bowl, beat together mashed sweet potatoes, sugar, salt, vanilla extract, butter and 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.