Place each banana on a 12 inch square of foil, crimp and shape foil around bananas so they sit flat. Cut each banana lengthwise about 1/2 inch deep, leaving 1/2 inch uncut at both ends. Gently pull each banana peel open, forming a pocket. Finely chop half of the candy bar. Fill pockets with chopped chocolate and 1/2 C. marshmallows. Grill the bananas, covered, over medium heat for 8-10 minutes or until marshmallows are melted and golden brown. Transfer bananas to serving plates, top with scoops of ice cream. Break remaining chocolate into pieces. Sprinkle choc. pieces and remaining marshmallows over tops. Garnish with whipped cream.