How to Make It
Preheat the oven to 350°. In a small skillet, toast the sesame seeds over moderate heat, stirring, until fragrant and golden, about 2 minutes.
Set out six 16-by-12-inch sheets of foil, shiny side down, with the shorter edges facing you. Spread about 1 teaspoon of the butter in the center of the bottom half of each sheet. Mound the sweet potatoes on the buttered areas and sprinkle with the sesame seeds, soy sauce, garlic and scallions. Season with salt and pepper and dot with the remaining butter.
Fold over the top of the foil to enclose the potatoes and crimp the edges to seal. Slide the 6 packets onto 2 large baking sheets and bake for 30 minutes, reversing the pans halfway through, until the packets are puffed and the potatoes are tender. Transfer to plates and serve.