Preheat oven to 350°F. In ... well combined. Beat butter until light and fluffy, gradually ... top of the cookies. Bake at 350°F 10-12 ... the M&M's, if desired.
In the large mixing bowl ... beat together sugar, butter, egg yolks, salt and ... the end. Arrange cookies on parchment-lined or ungreased baking ... thicker than for piping.