Special equipment: Heavy-duty foil
Prepare a grill for medium-high heat. Cut four 12-inch square pieces of heavy-duty foil.
Toss together the sugar, butter and salt in a small bowl.
Cut the peaches in half and remove the pits. Stuff each half with some of the brown sugar-butter mixture.
Put two peach halves, stuffed-side down, in the center of each piece of foil. Gather up the sides of the foil and crimp the edges tightly to make 4 purse-shaped packets.
Put the packets on the grill, close the lid and cook for 15 minutes, changing the position of the packets on the grill about half way through. (You can remove and open the packets to check their progress at any time. Be careful of the steam.) Remove from the grill and let rest for a few minutes.
Carefully open each packet (hot steam will escape) and use a spoon or fork to flip the peach halves over, they should be tender with a caramelized sauce. Top each serving with 1 tablespoon of granola and a scoop of frozen yogurt if desired.