Preheat oven to $375^{circ}text{F}$ ($190^{circ}text{C}$).
Roast or bake sweet potatoes until fork-tender; cool slightly, then scoop the flesh into a large mixing bowl. Discard skins.
Add all remaining base ingredients: melted butter, brown sugar, heavy cream, beaten eggs, vanilla extract, cinnamon, nutmeg, and salt.
Mash or mix the ingredients until they are just combined and the mixture is smooth and free of lumps. Do not overmix
Spread the mixture evenly into a lightly greased $9 times 13$-inch casserole dish.
Bake the base for 25 minutes at $375^{circ}text{F}$. Remove from oven and prepare for topping application (pecan streusel added now, marshmallows added later).
