Stir together the instant espresso and boiling water until smooth. Set aside.
Combine the cream and the corn syrup in a heavy saucepan and bring to a boil over medium heat. Remove the pan from the heat and stir in the espresso. Pour the mixture over the chocolate and let sit, covered, for 2 to 3 minutes. Stir until the chocolate is completely melted and the sauce is very smooth.
Keep warm until ready to use, or let cool to room temperature and store in a container with a tight-fitting lid in the refrigerator for up to 2 weeks. Reheat chilled sauce over very low heat on the stovetop or in the microwave until warm enough to pour easily.