Allow egg whites to stand at room temperature for 30 minutes.
Meanwhile, in a small saucepan combine sugar, cocoa powder, cornstarch, and coffee. Stir in milk all at once.
Cook and stir over medium heat until bubbly. Remove from heat. Stir in vanilla. Pour into a medium bowl. Cover surface of mixture with plastic wrap. Set aside.
In a medium mixing bowl beat egg whites and cream of tartar with an electric mixer on medium speed until stiff peaks form (tips stand straight).
Fold about 1/4 of the beaten egg whites into chocolate mixture to lighten. Fold in remaining beaten egg whites. Gently pour into 4 ungreased 1-cup soufflé dishes. Place soufflé dishes in a shallow baking pan.
Bake in a 375 degrees F oven for 15 to 20 minutes or until a knife inserted near the centers comes out clean. Serve immediately.