Add the salt and cream of tartar to the egg whites; beat with an electric mixer on high speed just until egg whites form stiff peaks when beaters are lifted. Stir about 1/4 of the egg whites into the chocolate mixture. Add remaining egg whites; to fold in, use a rubber spatula to gently cut down vertically through the mixture, then slide the spatula across the bottom of the bowl and up the side, turning mixture over. Rotate the bowl 1/4 turn, and repeat this down-across-up motion. Continue folding just until egg whites are blended into mixture. Carefully pour batter into the soufflé dish; use a rubber spatula to scrape batter from bowl, spread batter evenly in pan and smooth top of batter.