Set up the grill for indirect grilling and preheat to medium.
Cut 4 sheets of heavy-duty foil, each 14 X 8 inches. Place a sheet of foil, shiny side down, narrow edge toward you on the work surface. Smear a tablespoon of the butter in the center of the bottom half of the rectangle (the half closest to you).
Mix the soy sauce, sesame seeds, garlic and scallions.
Top each buttered area with 1/4 of the potatoes and 1/4 of the soy sauce mixture. Season with salt and pepper before closing up the packages.
Fold the top half of the foil over the potatoes and bring the top and bottom edges together. Fold the edges over several times to make a tight seal.
When ready to cook, place the foil packages in the center of the hot grate, away from the heat. Cover the grill and cook until the packages are dramatically puffed, 20 to 30 minutes.
Serve, warning everyone to open the packages at arm's length, using knives and forks, as the escaping steam will be very hot.