Prick sweet potatoes all over with a fork.
Microwave until cooked all the way through and no hard spots remain. About 8 - 10 minutes or so. ( Alternatively, can be baked in oven at 400* F for about 45 minutes to an hour - or until soft all the way through). Cool until you can easily handle them.
Slice open and scoop out insides of potatoes into a oven proof casserole dish.
Mash with a fork or potato masher until hard lumps are gone.
Add all the rest of ingredients and combine well.
Bake in oven at 350*F for about 15 minutes to heat through. Add marshmallows, if using, and cook a few minutes more.
***To brown marshmallows, turn oven to broil until marshmallows turn golden. Just a minute or two- don't let them burn!
**** This recipe can be made a day ahead and then reheated, adding marshmallows, if using, when ready to heat and serve.
