Place the sweet potatoes in a large saucepan and cover them with cold water. Bring to a boil and cook until very tender and easily pierced with a fork.

Drain the potatoes well and transfer them to a large bowl while they’re still hot. Add the cubed butter and mash until soft and smooth. The heat from the potatoes will melt the butter as you mash.

Add the brown sugar and cinnamon and mix until everything is well combined and there are no large pieces of sweet potato remaining.

Grease a 9-inch (23 cm) round baking dish with butter or cooking spray. Spoon in the sweet potato mixture and smooth the top with a spatula.

For the pecan topping, combine the melted butter, brown sugar, flour, and salt in a bowl.

Stir in the chopped pecans until you have a moist, crumbly mixture.

Sprinkle the pecan mixture evenly over the sweet potato filling.

Arrange the marshmallows in a single layer over the pecan topping.

Bake in a conventional oven preheated to 350°F (180°C) for 25–30 minutes, until the casserole is hot, the pecan topping is lightly golden, and the marshmallows are puffed and caramelized on top.

Keep a close eye on the casserole during the last few minutes because the marshmallows can go from golden to VERY golden quickly.

Let the casserole rest for 5–10 minutes before serving. Serve hot or warm.