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 minutes.
Roll dough to ¼-inch thickness and cut heart shapes with a 1- to 1½-inch heart cookie cutters. Bake on parchment-lined baking sheets for 8 to 10 minutes until slightly golden around edges. Cool completely.
Ice cookies with a thin icing tinted green and sprinkle with white crystal sanding sugar. Arrange cookies to form shamrock shapes, then paint icing stems on serving platter with a small brush.
Notes
Royal Icing
Make icing by mixing together 4 cups confectioners' sugar, 4 tablespoons 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!