This is wonderful as a plain cookie or dusted with confectioner's sugar. The Boyajian natural oils, while optional, make for a delicate but distinct citrus bouquet. Do not substitute generic extracts.
Citric acid adds a tang without changing the moisture balance of the recipe.
Worth a try. Cream the sugar with the butter, shortening and citrus zests until light and pasty in consistency. In a measuring cup, stir together the egg yolks, citrus oils, vanilla and cream. Blend into creamed butter mixture. In a separate bowl, stir together flour, salt, corn starch and baking powder.