Peel and cube sweet potatoes. Boil for around 12 minutes or until softened when poked with a fork.
Preheat oven to 350 degrees. Mash sweet potatoes and stir in remaining ingredients.
Add to a greased 9x13 baking pan.
Prepare topping: combine brown sugar and flour; cut in softened butter until the mixture resembles large crumbles.
Stir in the chopped pecans and sprinkle over the top of the sweet potatoes. Bake for 30 minutes.
If adding marshmallows; broil for around 1-2 minutes until lightly golden at end of cooking time.