Cream butter or margarine, oil and sugar until light and creamy. Add egg yolks, one at a time, beating well after each addition. Stir in lemon essence and naartjie peel. Sift flours and salt, and mix into creamed mixture to make a very soft dough. Roll out to 6 mm thick on lightly floured surface and cut out rounds with fluted biscuit cutter. Place rounds in greased patty pans.