How to Make It
Set a fine strainer in a bowl over a larger bowl of ice water. Microwave the chocolate and butter until melted. Let cool slightly.
In a saucepan, simmer 2 cups of the milk with 1/4 cup of the sugar and the salt. In a small bowl, whisk the cocoa powder and cornstarch with the remaining 1/3 cup of milk. Whisk the cocoa paste into the hot milk. In the same bowl, whisk the yolks and whole egg with the remaining 1/4 cup of sugar. Gradually whisk some of the hot milk mixture into the eggs to warm them, then whisk the egg mixture into the saucepan and cook over moderately high heat, whisking, until very thick, about 5 minutes. Remove from the heat and whisk in the chocolate-butter, vanilla and espresso. Strain the pudding into the bowl set in the ice bath and stir until cooled. Place a sheet of plastic wrap directly on the surface of the pudding and refrigerate until chilled, at least 4 hours or overnight. Serve with whipped cream.