Preheat oven to 450 (or wait until cookies are freezing)
If you know the creaming method- do that, then skip to the next section. Combine butter and sugar until well creamed. Add egg yolks one at a time, followed by vanilla. Slowly incorporate flour until it is well mixed. It should be very pasty at this point.
Pipe dough onto cookie sheet using a large star tip.
Freeze for 20 minutes.
If desired, dip cookies in sprinkles.
Bake at 350 degrees for 18-20 minutes.
Remove from oven and cool on cooling rack- not on pan!