In a medium saucepan, mix together sugar, cocoa, cornstarch, and salt.

Turn heat to medium, and gradually stir in milk. Cook, stirring and scraping the bottom constantly, until the mixture begins to thicken and boil.

Boil for 1 minute, then remove pan from heat.

Place egg yolks in a small, heatproof bowl. Slowly add 1 cup of the hot mixture to the egg yolks, stirring constantly. (This will temper the egg yolks so they do not cook instantly when added to the pudding.)

Pour egg mixture into the hot pudding.

Return to heat and boil 1 minute, stirring and scraping the bottom of the pan constantly to prevent sticking and burning.

Serve immediately or let cool and serve chilled.