Cream butter and sugar until fluffy. Add egg yolks and beat until well mixed. Sift flour, set aside.
Beat egg whites in a separate bowl, until stiff. Alternately add a little of the flour and a little of the egg whites until all is added to the creamed mixture. Beat well. Pour in extract and beat again.
Bake in 9x9 square pan at 350 degrees F for 45 minutes or until done when tested, toothpick comes out clean.