Preheat oven to 350 degrees and wrap sweet potatoes in foil.  Place wrapped sweet potatoes on a lined baking sheet and bake for approximately 1 hour.  After 1 hour test sweet potatoes by piercing with a fork, if you can pierce them with a fork easily, they are done baking.  Allow to cool until you can handle them, remove foil, and remove skins by simply pushing off the skins from the meat of the potato.  Place cooked potatoes into a large bowl.

Mash sweet potatoes together with 1/4 stick of butter using a blender, or hand masher.   Process sweet potatoes until they are mostly smooth.  Add in 2 eggs, 1 teaspoon salt, 1 teaspoon ground cinnamon, 1/2 teaspoon nutmeg, and 1/2 teaspoon vanilla and blend until you have a uniform mixture.  Add 1/2 cup dark brown sugar and heavy cream to sweet potatoes and mix in brown sugar and heavy cream.

To make the "oatmeal cookie", or struesel topping combine together in a medium sized bowl 1/2 cup flour, 1 cup dark brown sugar, 1 cup quick cooking oats, 1/2 teaspoon cinnamon and 1/4 teaspoon salt.  Stir the dry ingredients together.  Add 1/2 cup softened butter, cut in butter with a fork until you have a crumbly mixture.

Assemble casserole by spreading the sweet potatoes into a 9 x 13 inch baking  pan.  Top with 2 cups marshmallows, and then place the oatmeal topping on top of the marshmallows.  Bake at 350 degrees for 30-45 minutes, this is done when the marshmallows have melted.