Preheat oven to 350 degrees. Place the flour, cocoa, and 1/4 cup of the sugar in a sifter, sift into a medium bowl. Repeat the sifting 5 times. Set aside. Sift the remaining 1 cup of sugar into a seperate bowl. Set aside. Place the eggwhites in a large bowl. Using an electric mixer, beat until foamy. Add the cream of tartar and beat until the egg whites form stiff peaks but are not dry. Fold in the 1 cup of sugar, 1 tablespoon at a time. Add the vanilla and lenon extract. Slowly sift small amounts of the flour mixture over the batter and fold it in until all flour is incorporated. Pour the batter into an ungreased 10" straight-sided tube pan. Bake 45 minutes. Remove from the oven and turn upside down to cool. Let stand for 11/2 hours. (Invert pan over kneck of a sturdy bottle to prevent cake from shrinking as it cools.) When the cake is fully cooled, run a knife around the edges of the pan to remove cake.