Line 13×9-inch pan with foil; grease foil. Place 15 graham squares on bottom of pan, cutting to fit if necessary. Coarsely crush remaining graham squares; set aside. Microwave chocolate and 2 Tbsp. of the butter in microwaveable bowl on HIGH 2 min.; stir until chocolate is completely melted and mixture is well blended. Spread half of the melted chocolate in thin layer over grahams in pan.
Microwave remaining 4 Tbsp. butter in 4-qt. microwaveable bowl on HIGH 45 sec. or until melted. Add miniature marshmallows; toss to coat. Microwave 1-1/2 min. or until marshmallows are completely melted, stirring after 45 sec. Add cereal; mix well. Place over chocolate-coated grahams; press gently to form even layer.
Top with large marshmallow halves, cut sides down. Sprinkle with crushed grahams; drizzle with remaining melted chocolate. Cool. Cut into 30 squares. Makes 2-1/2 doz. squares.