Preheat oven to 400.
Line baking sheet with foil.
Wash and pierce sweet potatoes. Then place on prepared pan.
Bake for 1 hour.
Remove skins from potatoes and discard.
Place potatoes in large bowl.
Mash or beat potatoes.
Add 6 tablespoons butter and beat until creamy.
Add Vanilla, 1 cup brown sugar, 1/2 cup Half and Half, 1/2 teaspoon Cinnamon and 1/2 teaspoon Nutmeg.
Beat until well blended.
Preheat oven to 350.
Heavily butter 2 quart casserole and add sweet potato mixture.
Sprinkle marshmallows on top.
In a small bowl, melt 4 tablespoons butter in microwave.
Add flour, 1/3 cup brown sugar, quick oats, 1/4 teaspoon Cinnamon and 1/4 teaspoon Nutmeg.
Mix together with a fork until crumbly.
Sprinkle mixture over marshmallows.
Bake 30-35 minutes.