Sift together flour, salt, cream of tartar and cornstarch and set aside. Beat egg whites at high speed. When they begin to thicken, slowly add sugars and whip to medium stiffness. Add vanilla extract. Stir lemon zest and poppy seeds into dry mixture, then gradually fold into meringue. Pour into two ungreased loaf or tube pans. Bake at 325 degrees for about 50 minutes. (A toothpick should come out dry, with a few crumbs.)
Remove the cakes from the oven and let them cool upside-down on a rack. Serve with a sauce made from seasonal fruits.
Chef Pleau likes strawberry-rhubarb, which he makes with strawberries, rhubarb, sugar and a little water. While the sauce is cooking, sample it?adding more fruit if it tastes too sweet, more sugar if seems too tart.