1In medium bowl, slightly beat eggs and egg whites; set aside.
2In 2-quart saucepan, mix sugar, cocoa and cornstarch. Gradually stir in milk. Cook over medium heat, stirring constantly with wire whisk, until mixture thickens and boils. Boil and stir 1 minute.
3Gradually stir at least half of the hot mixture into the eggs, then stir back into hot mixture in saucepan. Cook over low heat, stirring constantly, until thickened; remove from heat. Stir in butter and vanilla.
4Spoon into six 1/2-cup custard cups or ramekins. Top with marshmallows. Serve warm or cold.