Roast or boil—both work fine. Roasting brings out deeper flavor, but boiling is faster. Once they’re tender, scoop out the flesh and let it cool slightly.
Mash the potatoes until smooth. Stir in butter, brown sugar, milk, eggs, vanilla, spices, and salt. Mix until creamy and well combined. Pour into a greased baking dish.
In a bowl, combine pecans, brown sugar, flour, and melted butter. Stir until clumps form—the good kind.
Scatter the topping evenly over the casserole and bake at 350°F for 30–35 minutes, until the center is set and the top is crisp and golden.