1Preheat over to 400 degrees.
2Cream butter in a mixer on medium speed. Put egg yolks through a sieve and add to butter in mixer. Add sugar and vanilla. Add flour. Mix to incorporate.
3Roll into 1 inch balls and put on cookie sheet 2 inches apart. Depress each with a fork in a criss cross pattern.
Optional: I decorate these with green and red sprinkles for Chrismas before baking.
4Bake at 400 degrees for 15 minutes.