Line 8-inch square pan with foil, with ends of foil extending over sides. Mix Crust Mix, butter and sugar until blended; press onto bottom of prepared pan.
Beat milk and Filling Mix with mixer on low speed 30 sec. Beat on medium speed 3 min. (Filling will be thick.) Spread crust with half the filling. Blend remaining filling with half the chocolate; spread over layer in pan. Drizzle with remaining chocolate.