Preheat oven to 325. Beat egg whites until frothy; add salt, cream of tartar and lemon oil and vanilla extract. Continue beating until they form stiff but not dry peaks. Using a rubber spatula, gradually fold in the sugar a few tablespoons at a time. Using the same rubber spatula, gradually fold in the flour, one quarter at a time. Be sure that you use a true folding motion, gently turning the spatula from the bottom of the bowl to the top, folding in air with each stroke as opposed to beating or stirring. You just went to all that work the whip up the egg whites, don?t want to beat them to death. Pour batter into an ungreased, 9-inch angel food cake pan (you know the one with the tube in the center ? not a bundt pan). Bake one hour or until top is golden brown and the cake pulls away from pan sides. Remove cake from oven . Cool in the pan ten minutes and then invert pan and let cake stand upside down for an hour or until cooled. You might let the cake stand over a plate in case your oven was funky and the cake plops out of the pan prematurely. Hey, it happens. While cake is cooling prepare the glaze. In a small bowl, whisk together confectioners sugar, lemon juice and lemon zest. Let stand ten minutes before glazing the cake as it thickens a little. Serve immediately.