Allow egg whites to stand at room temperature for 30 minutes.
Meanwhile, in a saucepan combine the sugar, cornstarch, cocoa powder, and coffee crystals. Stir in milk all at once. Cook and stir over medium heat until bubbly; cook and stir for 2 minutes more. Remove from heat.
Stir in liqueur and vanilla. Pour into a large bowl. Cover surface of mixture with clear plastic wrap. Set aside.
In a large mixing bowl beat egg whites and cream of tartar until stiff peaks form (tips stand straight). Fold about one-fourth of the egg whites into the chocolate mixture to lighten. Fold in remaining egg whites.
Gently pour mixture into an ungreased 2- to 2-1/2-quart souffle dish.
Bake in a 375 degree F. oven for 20 to 25 minutes or until a knife inserted near the center comes out clean. Serve immediately. Makes 6 servings.