CRUSH 12 grahams to form fine crumbs; mix with butter. Press onto bottom of 13x9-inch pan. (To crush graham crackers: place graham crackers in resealable plastic bag. Flatten bag to remove excess air, then seal bag. Crush the grahams into crumbs by rolling a rolling pin across the bag.)
CHOP remaining grahams; sprinkle over graham crust. Top with chocolate chunks and nuts. Pour milk evenly over ingredients in pan.
BAKE 25 to 30 min. or until lightly browned. Sprinkle with salt; cool.
MELT 1 oz. semi-sweet chocolate as directed on package; drizzle over dessert. Refrigerate 5 min. or until chocolate is firm.