Heat oven to 350°F. In a bowl, beat butter until smooth. Add confectioners' sugar and blend together. Beat in egg yolks, vanilla and salt. Stir in flour until completely incorporated. Wrap and chill dough about 30 min.
Roll dough to ¼-inch thickness and cut heart shapes with a 1- to 1½-inch cookie cutters. Bake on parchment-lined baking sheets for 8 to 10 min until slightly golden around edges. Cool completely.
Ice cookies with a thin icing tinted green and sprinkle with white crystal sugar. Arrange cookies to form shamrock shapes, then paint icing stems on serving platter with a brush.
Make icing by mixing together 4 cups confectioners' sugar, 4 tbsp meringue powder and ⅓ cup water. Mix until icing is the consistency of cold honey. Cover or use immediately.And for the stems! A small brush of icing on the plate will make the cute shamrock stems. And for the lucky ones, a four-leaf clover instead of three!