Preheat oven to 350 degrees. Cream butter and egg yolk. Add powdered sugar, flour and vanilla and mix well by hand. Make into little balls and press down with a fork using powdered sugar. Place onto an ungreased cookie pan. Bake in preheated oven for 8 - 10 minutes. * May use Hershey's Kisses, whole walnuts or colored sprinkles on top of each cookie before you place them in the oven. ** May dip in powdered sugar after they cool.