Beat butter at medium speed until smooth. Add sifted confectioners sugar and beat again until smooth. Beat in egg yolks and salt. Reduce speed to low and add cocoa and flour just until mixed.
Gather dough into a ball on your counter and divide it in half. Wrap each half in saran wrap and chill in fridge for 30 minutes.
Form each half of dough into a log about 1 1/4 inches thick and place back in fridge for 2 hours. At this point you can keep dough in your fridge up to 3 days or store in your freezer for a month.
Slice log into cookies about 1/3 inch thick (or thicker if you'd like). Bake on an ungreased cookie sheet in a 350 degree oven for 12-14 minutes, or until cookies are set. Cool before serving.