Place 1 graham square on microwaveable plate; top with marshmallows and chocolate. Microwave on HIGH 10 sec. or until marshmallows begin to puff; cover with remaining graham square.
Heat grill to medium heat. Fill graham squares with marshmallows and chocolate; wrap in foil. Grill 4 to 5 min. or until marshmallows begin to melt.
Heat toaster oven to 350F. Fill graham squares with marshmallows and chocolate; place on foil-covered tray.