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 morsels. Shape dough into 2 round discs; wrap in plastic wrap. Refrigerate for about 1 hour or until firm.
PREHEAT oven to 350° F.
ROLL each disc to 1/4-inch thickness between two sheets of waxed paper. Cut into shamrock shapes; place on ungreased baking sheets.
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; sprinkle with coarse sugar.