Cream together the butter and sugar until light and fluffy.

In a separate bowl, combine the flour and grated hard-boiled egg yolks.

Gradually add the flour mixture to the butter mixture, mixing until just combined.

Stir in the vanilla extract.

Chill the dough until firm, about 30 minutes.

Preheat oven to 340 degrees F (170 degrees C).

Load the chilled dough into a cookie press.

Press out cookies onto ungreased baking sheets.

Decorate as desired.

Bake for 10 minutes, or until lightly golden around the edges.

Let cookies cool completely on baking sheets before serving.