Preheat over to 375.
Set the egg whites aside to bring to room temp.
Sift together into a small bowl the powdered sugar, potato starch and corn starch - the sifting is essential in this recipe. Measure the granulated sugar to have handy.
In a large mixing bowl, place the egg whites, cream of tartar, salt and flavoring. With mixer on high, beat until the mixture is well blended and foamy. Continue to beat while adding the granulated sugar slowly. Beat just until sugar is dissolved and whites form stiff peaks.
With a rubber spatula, gently fold in the flour mixture about a quarter at a time, folding just enough so the flour disappears.
Pour batter into an ungreased 9 inch tube pan and cut through with a spatula to break any bubbles.
Bake at 375 for 35 minutes, or until top springs back when lightly touched. Remove from oven and cool cake inthe inverted pan. Remove cake when completely cool.