Cream together butter and sugar. Add egg yolks and mix well. Combine flour, baking powder and salt. Add lemon juice and flour mixture to creamed mixture. Beat egg whites until soft peaks form. Fold into batter with lemon zest. Fill paper-lined muffin cups two-thirds full. Bake at 350 degrees for 20-25 minutes or until light golden brown or a tooth pick inserted comes out clean. Mix together glaze ingredients adding more sugar if needed to thicken or more milk if needed to make more thin. Pour drizzle over muffins.