Line a 13x9" baking pan with parchment paper, extending paper up over 2 sides of pan; set aside. In a large mixing bowl, combine butter and sugar; stir in flour. If dough is crumbly, mix in 1-2 tbsp. additional softened butter. Press dough evenly into baking pan; bake in a preheated 350° oven until lightly brown (about 20 min.). Cool in pan for 10 min. Using a 2 ½" or smaller heart-shaped cookie cutter, press into shortbread. Carefully lift shortbread out of pan with parchment paper and place on a cooling rack; cool completely. Melt dipping chocolate according to pkg. directions. When bars are completely cool, dip half of each shortbread heart into the chocolate; place on parchment paper to set. Store in an airtight container with parchment paper between layers.