Preheat your oven to 180°C (350°F). Gather your baking dish to assemble the casserole.
Place the peeled and chopped sweet potatoes into a large pot and cover with salted water. Bring to a boil and cook until tender, about 15 minutes. The sweet potatoes should feel soft when pierced with a fork.
Drain the sweet potatoes thoroughly and transfer them to a mixing bowl. Mash until smooth and creamy, with no lumps remaining. The mash should be fluffy and warm to the touch.
Add the melted butter, brown sugar, and vanilla extract to the mashed sweet potatoes. Mix well until all ingredients are fully incorporated and the mixture is glossy and smooth.
Spread the sweet potato mixture evenly into a buttered baking dish, smoothing the top with the back of a spoon or spatula. The surface should be even and slightly shiny.
In a small bowl, combine chopped pecans, flour, and cinnamon. Sprinkle this crunchy topping evenly over the sweet potato layer, creating a textured, cinnamon-scented crust.
Melt the remaining butter in a small saucepan and stir in mini marshmallows until melted and smooth. Carefully spread this gooey mixture over the pecan topping, creating a glossy, marshmallow layer.
Bake in the preheated oven for 25-30 minutes, or until the topping is golden and bubbling. The marshmallows should be melted and slightly browned, with a crackly surface.
Remove the casserole from the oven and let it rest for 5 minutes. The topping will firm up slightly and the flavors will meld beautifully.
Serve warm, with the glossy marshmallow peaks and crunchy pecan topping creating a delightful contrast. Enjoy this festive, comforting dish that’s perfect for special occasions.
