Preheat the oven to 325 degrees.
Arrange the sweet potatoes on a foil-lined baking sheet. Using a knife, poke several holes in each of the potatoes to allow the steam to escape. Once the oven has reached the correct temperature, place the baking sheet in and bake for approximately 45 minutes or until soft. Allow to cool, once handleable, peel and discard the skins.
Raise the oven temperature to 350 degrees.
Using an old fashioned potato masher, mash the potatoes until you have achieved a smooth consistency. Switch to a large whisk, and incorporate the eggs, one at a time. Whisk in the brown sugar, then the sour cream, heavy cream, and mascarpone, mixing well between each addition. Add the seasonings, thyme, black pepper, and 2 teaspoons kosher salt. Once combined, scrape into a 9 X 7 X 2 casserole dish. Cover with foil and bake for 30 minutes. Remove the foil and bake for an additional 15 to 20 minutes or until the center is just set.