Peel and cut the sweet potatoes into large chunks and place them in a large saucepan. Cover with water and bring to a boil. Cook the sweet potatoes until they are fork-tender and fall apart easily when pierced with a fork.

Drain the cooked sweet potatoes and return them to the saucepan. Mash thoroughly with a potato masher. Add melted butter, brown sugar, salt, ground cinnamon, vanilla extract, and ground nutmeg. Mix until everything is well combined into a smooth filling. I like to adjust the cinnamon to taste here for a warmer flavor.

In a separate bowl, combine melted butter, brown sugar, all-purpose flour, salt, and finely chopped pecans. Stir together until thoroughly mixed and crumbly.

Lightly grease a 9x13-inch baking dish with cooking spray. Spread the sweet potato filling (from Step 2) evenly into the prepared dish. Sprinkle the pecan topping (from Step 3) over the sweet potato layer. Finally, top with an even layer of miniature marshmallows.

Bake in a preheated oven at 350°F (175°C) for 25-35 minutes, or until the filling is bubbly and the marshmallows are golden brown on top. Watch closely towards the end to prevent the marshmallows from burning. For an even richer flavor, I like to toast the marshmallows for an extra minute at the end under the broiler—just be sure not to walk away!