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 1/2 cups morsels. Divide dough in half; cover. Chill for 1 hour or until firm.
ROLL half of dough to 1/4-inch thickness between two sheets of waxed paper. Remove top sheet of waxed paper. Cut into shapes using cookie cutters. Lift from waxed paper; place on ungreased baking sheets. Chill for 10 minutes. Repeat with remaining dough.
BAKE in preheated 350° F. oven 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 additional 10 seconds; knead until smooth. Cut tiny corner from bag; squeeze to pipe chocolate.
Makes about 3 dozen cookies.