Twice-Baked Sweet Potatoes

14 sweet potatoes
1/2 cup sour cream

6 tablespoons unsalted butter, softened, plus 6 cold unsalted butter, thick slices
1 3/4 cups packed light-brown sugar
4 teaspoons lemon juice 1/2 teaspoon nutmeg
Salt and freshly ground pepper 2/3 cup pecans, toasted, coarsely chopped
40 large marshmallows, halved crosswise or minis

Preheat oven to 400 degrees. Prick sweet potatoes with a fork. Bake until soft, about 50 minutes. Let cool slightly on a rack.

Cut a lengthwise slit in potatoes almost all the way through, leaving about 1 inch at each end intact. Scoop out inside, leaving a 1/2-inch border on skins. Put potato scoopings in a medium nonreactive bowl, then transfer half to a food processor. Process until smooth; return to bowl. Stir in sour cream, 6 tablespoons softened butter, 1 cup brown sugar, lemon juice, and nutmeg. Season with salt and pepper.

Reduce oven temperature to 350. Transfer sweet potato mixture in batches to a pastry bag fitted with a very large tip (or plastic bag). Pipe mixture into 12 of the potato shells, dividing evenly; discard remaining 2 shells. Filled shells can be refrigerated in an airtight container up to 1 day. Just before serving, top with remaining 3/4 cup brown sugar, the sliced butter, and pecans; arrange marshmallows on top. Bake on a rimmed baking sheet until hot and marshmallows are golden brown, 7 to 10 minutes.