Combine the milk and cocoa in a pan. Cook over medium heat until it just comes to a simmer. While the milk mixture is warming up, combine the sugar, cornstarch, and salt in a medium sized bowl. Whisk the egg yolks into the cornstarch mixture. When the milk is hot, gradually pour about ½ cup of the milk mixture into the egg yolks, stirring constantly until smooth. Pour the egg mixture back into the pot and stir until smooth. Cook over medium heat until the mixture becomes very thick.