In a medium saucepan, whisk together the cornstarch, cocoa powder, sugar salt, and salt.In a separate bowl, combine the milk and egg.Gradually pour the milk mixture into the saucepan, whisking until thoroughly mixed.Heat the mixture, stirring constantly, over medium heat until it boils. Cook for 1 minute, then remove from heat.Stir in butter and vanilla extract.Pour the pudding into the suitable molds rinsed with cold water (the pudding can be taken out more easily).Chill.
