BEAT butter, brown sugar, granulated sugar, vanilla extract and salt in large mixer bowl until creamy. Beat in egg yolk. Gradually beat in flour. Stir in 1 ½ cups morsels. Divide dough in half. Cover; refrigerate for 1 hour or until firm.
PREHEAT oven to 350° F.
ROLL half of dough to ¼-inch thickness between two sheets of wax paper. Remove top sheet of wax paper. Cut dough into shapes using cookie cutters. Lift from wax paper; place on ungreased baking sheets. Refrigerate for 10 minutes. Repeat with remaining dough.
BAKE for 9 to 11 minutes or until golden brown. Cool on baking sheets for 2 minutes; remove to wire racks to cool completely. Spread with frosting; pipe with melted chocolate.
TO PIPE CHOCOLATE: Place remaining morsels in heavy-duty plastic bag. Microwave on HIGH (100%) power for 45 seconds; knead. Microwave for 10 seconds; knead until smooth. Cut tiny corner from bag; squeeze to pipe chocolate.