Start by peeling the sweet potatoes, then chop them into large chunks and transfer to a large saucepan. Add water to cover and bring to a boil.

Let the potatoes boil until they are fork-tender and can be easily pierced; they should be very soft and almost falling apart.

Drain the potatoes and put them back into the saucepan. Add the stick of butter and mash using a potato masher; the residual heat will melt the butter. Then incorporate the rest of the filling ingredients: brown sugar, salt, cinnamon, vanilla extract, and nutmeg. Stir until everything is well mixed.

To make the pecan topping, combine the melted butter, brown sugar, flour, salt, and pecans in a separate bowl, stirring until uniform.

Coat a 9x13-inch baking dish with cooking spray.

Evenly distribute the sweet potato mixture across the bottom of the dish.

Evenly sprinkle the pecan topping over the sweet potato layer.

Cover the surface with an even layer of mini marshmallows.

Bake in an oven preheated to 350°F (175°C) for 25-35 minutes, until the mixture is bubbly and the marshmallows turn golden brown. Keep a close eye on it near the end to prevent burning.