Combine coffee, 2/3 cup sugar and pinch salt in top of double boiler (or saucepan over low heat). Dissolve gelatin in milk and add to coffee mixture while it heats.
Beat egg yolks and scant 1/2 cup sugar. Temper with a few spoonsful of hot coffee mixture and add to the double boiler. Cook at no higher than a simmer for a few minutes.
Beat egg whites until stiff but not dry. Off heat, fold into the liquid to thoroughly but gently.