Turn the oven up to 400 F. Place the egg yolks in a small bowl.
Combine sugar and cornstarch in a medium saucepan.
Add water and place over medium heat.
Stir constantly until the mixture boils. The mixture will start to look clear and glossy and will feel a bit thicker.
Stir while the mixture boils for one minute.
Pour approximately half the mixture into the egg yolks and stir till combined.
Pour the egg yolk mixture back into the pan with the remaining half of the sugar/cornstarch mixture and stir constantly over medium heat for an additional 2 minutes (mixture should still be boiling).
Remove from heat and mix in the butter, espresso powder, and chocolate syrup until the butter is melted.
Pour the mixture into cooled pie crust.