Cream the butter and sugar for about 2 minutes, then add the egg yolks till fully incorporated. Toss in the citrus zest. Stir the flour and corn meal in with a wooden spoon, kneading a few times till the mixture pulls together. Let sit for one hour; this will help the corn meal to absorb moisture.
Preheat oven to 325 degrees F. Prepare a baking sheet with lined parchment paper or non stick baking spray.
Pull the dough into 1-1/2 tablespoon size chunks and roll between you fingers into little logs. Place them evenly spaced on the cookie sheet and run the tines of a fork firmly but gently over the surface. Patch together any bits of dough that come off each cookie.
Bake half the dough at a time, for about 16 to 18 minutes, until just golden at the edges. Cool on a wire rack. These cookies are best after they have completely cooled. Store in a cookie jar.
Yields about 18 biscotti. This recipe can easily be doubled.