Whip egg whites on high speed in a mixer and add cream of tartar. Whip until white and foamy and the peaks are fairly stiff. Citric acid/any other acid could be used in place of cream of tartar. The acid isn't needed, but helps to stabalize the foam.
Dissolve the sugar in the water and heat on a thick-bottomed sauce pan. Allow it to boil until it thickens significantly. It shouldn't brown or burn, but it should be close to that state. It should be in the "soft ball" state.
Turn mixer back on high with the egg whites and slowly add the sugar mixture to this.
Melt the butter, add the chocolate chips, cocoa and salt to a sauce pan. Heat on low until everything is melted.
Slowly fold in the sifted flour. Mix until just incorporated.
Slowly fold the egg white mixture into the batter. Do not over mix this.
Pour the batter into a greased 13"x9" pan.
Bake at 350F until it passes the toothpick test. I actually prefer mine under cooked.