Heat broiler. Break the crackers in half to form squares. Place 8 cracker squares on a baking sheet and top each with a marshmallow. Broil until the marshmallows are puffed and lightly browned, about 1 1/2 minutes. Sandwich with the chocolate pieces and remaining crackers. Serve immediately.
Tip: Add a little zing to the Mexican S'mores for the adults: Substitute spiced chocolate bars (try the version from Vosges or Green & Black's) for the plain milk chocolate.