Cut the stem off each banana, keeping the peel intact. Slice the peel from stem to end, leaving a small amount uncut on each end. Leaving the bananas in their peels, carefully cut them into slices without puncturing the skin. Spray 6 banana-size strips of aluminum foil with nonstick cooking spray and place a banana on each strip. Gently stuff equal portions of marshmallows and chocolate chips into each banana. Wrap the foil around the bananas tightly and grill for approximately five minutes, or until the chocolate and marshmallows are melted. Peel open and eat with a spoon.