Combine cocoa powder, cornstarch, sugar substitute and sugar in a small saucepan. Add just enough of the milk to make a smooth paste, about 2 ounces. Gradually stir in remaining milk, stirring until smooth. Cook over medium heat, stirring constantly, until mixture begins to thicken, about 1 to 2 minutes. Remove from heat and stir in vanilla. Set aside to cool.

Separate the white and yolks of the eggs and discard the yolks. Beat egg whites to a stiff peak. Mix one large spoonful of egg whites into the cooled chocolate to lighten the mixture.  Fold the chocolate into the egg whites.  Spoon into a dessert dish and serve or refrigerate until needed.  Makes 1 serving.