Chocolate Pastry Cream

In a heatproof bowl whisk together yolks, sugar, cornstarch, cocoa powder, and salt (mixture will be very stiff). In a heavy saucepan bring milk just to a boil over moderate heat and in a stream add 1/4 cup to egg mixture, whisking until smooth, and whisk in remaining 3/4 cup milk. Transfer egg mixture to pan and bring to a boil, whisking (mixture will look curdled but will become smooth as whisked). Boil pastry cream, whisking vigorously, 1 minute and remove from heat. Stir in chocolate and butter, stirring until melted and combined well. Transfer hot pastry cream to a heatproof bowl and chill, surface covered with plastic wrap, until cold, about 2 hours. Pastry cream may be made 2 days ahead and chilled, covered.