Preheat oven to 350°F (180C). Using a medium bowl, dissolve cocoa in coffee; set aside to cool at room temperature. Combine 1/2 cup sugar, the flour and salt. Place egg whites in large mixing bowl and set it over a larger bowl of hot water. Stir for several minutes until egg whites come to room temperature. Beat egg whites until frothy. Add cream of tartar and continue beating until soft peaks form. Gradually add remaining sugar and beat until stiff peaks form. Do not over beat. Sift the flour mixture over the beaten egg whites in 3 parts, folding in gently after each sifting. Stir approximately 1 cup of egg white mixture into the coffee mixture. Fold it back into the egg white mixture until combined.
Pour the batter into a spring-form cake pan. Smooth the top and run a small knife through the batter to remove any air pockets.
Bake for 50-60 minutes, or until the top springs back when lightly touched. Allow cake to sit in pan until nearly cool. With a knife, loosen the edges and invert the cake onto a serving platter.