Leave the banana in the peel. Hold lengthwise. Slice through the skin and halfway through the banana end to end. Make same incision about 1/2 an inch away. Peal back the sliced peel, leaving the skin still attched. Carve out trench in middle of banana. Fill with one tablespoon each marshmellows and chocolate chips. Replace peel. Hold "trench-side up" and place on tin foil. Wrap banana lengthwise - keeping the fold of the foil over the trench. (this makes it easier to check if it is done) Place on hot coals or near fire. You know it is done when the chocolate chips are starting to melt.