Combine espresso powder, water, and corn syrup in a small heavy saucepan, and bring it to a boil, stirring.
Remove from heat and stir in vanilla, chocolate, and salt. Stir until chocolate is melted and sauce is smooth.
Sauce can be made 3 days ahead and kept chilled, covered. Warm over low heat or on low for a few seconds in the microwave before serving. Makes about 3/4 cup of sauce, so I figured about 6 servings at 2 tablespoons each