Heat a grill to medium (350 degrees F). Leave bananas in their skins and make 1 slit down the length of each banana. Pack the slits with brown sugar.
Put the banana(s) on the grill and grill until soft and charred, about 5 minutes. Remove them to a cutting board and set aside to cool slightly. Peel the skins from the bananas and cut into 1/2-inch thick slices. Meanwhile, put the marshmallows on skewers and toast over the grill until brown and melty, about 2 minutes. You can also brown the marshmallows using a hand-held torch.
To assemble, spread some peanut butter on graham crackers and top with bananas and marshmallows. Cover with another cracker and serve. Enjoy!