Preheat the oven to 375 degrees F. Line a 9x5 inch or slightly smaller metal loaf pan with regular foil.
Sift together the flour, cocoa powder and half of the ground sugar.
In a mixing bowl using an electric mixer, beat the egg whites until foamy. Continue beating and add the cream of tartar. When soft peaks form, begin adding the sugar gradually until peaks are just stiff enough to hold – don’t overbeat. Beat in the salt and vanilla.
Sift about a third of the flour mixture over the top of the egg white mixture and fold it in with a big rubber scraper. Continue adding the flour mixture, folding until incorporated.
Carefully scrape into the loaf pan. It should come just about to the top.
Bake at 375 degrees F. on center rack for 28 minutes.
Let cool completely. Lift from pan, carefully peel away the foil, then slice with a serrated knife