In a large pot, combine peeled sweet potato cubes with cold water, bring to a boil, then simmer for 15–20 minutes until tender.

Drain potatoes thoroughly, then mash with softened butter, brown sugar, vanilla extract, cinnamon, and a pinch of salt until silky smooth.

Set the oven to 350°F and center a rack.

Spoon the mashed sweet potatoes into a greased 9×13-inch baking dish, smoothing the top evenly.

In a medium bowl, toss chopped pecans, brown sugar, and flour until evenly coated; cold butter pieces should cling to each cluster.

Evenly sprinkle the nutty topping over the sweet potato layer, pressing gently to form clusters.

Bake at 350°F for 25–30 minutes until the topping is golden brown and crisp.

Optional: Top with mini marshmallows in the last 5 minutes for extra gooey sweetness.