PREHEAT oven to 375° F. Grease and wax paper-line 8 1/2 or 9-inch heart-shaped pan.
COMBINE flour, baking soda and salt in small bowl. Beat butter, brown sugar, granulated sugar and vanilla extract in large mixer bowl until creamy. Beat in egg. Gradually beat in flour mixture. Stir in 1 cup morsels; spread into prepared pan.
BAKE for 20 to 30 minutes or until golden brown. Let cool in pan for 10 minutes; remove to wire rack to cool completely.
MICROWAVE remaining morsels in small, microwave-safe bowl on MEDIUM-HIGH (70%) power for 1 minute; STIR. Morsels may retain some of their shape. If necessary, microwave at additional 10- to 15-second intervals, stirring until melted; spread over cookie. Decorate with icing.