preheat oven to 400° F. Line a baking sheet with aluminum foil and poke holes in the sweet potatoes using a fork. Place on a prepared baking sheet and bake until very tender, about 1 hour. Remove from oven and let cool for 30 minutes. Reduce oven temperature to 350° F.

While the potatoes are cool, cook the carrots in boiling water for 15-20 minutes or until very tender; drain and set aside.

In the bowl of a food processor, process carrots with the butter until smooth, scraping down the sides as needed. Transfer carrot mixture to a large bowl.

Scoop out flesh of the cooled potatoes. Process each batch of sweet potatoes with a little sour cream until the potatoes are very smooth. Remove to the bowl with carrots and add any remaining sour cream.

Stir in brown sugar through black pepper and stir to combine. Spoon mixture into a lightly greased 13- x 9-inch baking dish. Bake at 350° F for 30 minutes. Remove from oven and top with the marshmallows. Bake 10 more minutes or until marshmallows are golden brown. Let cool slightly before serving.