Preheat your oven to 375°F. Peel the sweet potatoes and cut into cubes.
Boil the sweet potatoes in a pot of water for 15 minutes until tender. Drain and set aside.
In a large bowl, mash the sweet potatoes along with the brown sugar, butter, cinnamon, and vanilla extract, and season with a pinch of salt and pepper.
Spread the mashed potatoes into a baking dish or casserole and sprinkle with the marshmallows and the pecan nuts.