Cream butter and sugar until light and fluffy. Add yogurt and beat well. Add egg yolks and lemon rind; mix well. Blend in flour sifted with baking soda. Beat egg whites until stiff and fold gently into cake batter. Pour into a greased and floured, 10 cup mold or a 9 x 13" pan. Bake at 350* for 30 minutes or until toothpick comes out clean when inserted into center of cake. No frosting is needed.