1) Place the cocoa, cornstarch and water in a heavy-bottomed sauce pan over medium heat. Stir everything together; don't worry about a few small lumps.
2) Whisk in the sweetened condensed milk and egg yolks. Since the mixture is barely warm, there's no need to worry about the yolks cooking.
3) Cook the pudding for 4 to 5 minutes, or until it's thick and creamy. Remove it from the heat and immediately stir in the butter, vanilla and espresso powder. The pudding will become very smooth and glossy.
4) If desired, press the pudding through a sieve to remove any lumps. Place a piece of plastic wrap directly on top of the pudding to prevent a skin from forming, and chill for several hours.
5) To use as a pie filling, place the cooked filling in a pre-baked 9" pie shell and chill until firm.
Yield: approximately 3 cups pudding or pie filling.