Nutrition per serving may change if servings are adjusted.
2½ ounces bittersweet chocolate, chopped into ¼-inch pieces
6½ tablespoons unsweetened cocoa powder
⅓ cup granulated sugar, (use 1-2 tablespoons more for a less-bittersweet sauce)
¼ teaspoon instant coffee granules
1 cup hot water
⅓ cup dark corn syrup
2½ teaspoons vanilla extract
Combine chocolate, cocoa, sugar and coffee granules in a food processor. Process until finely ground, about 1 minute.
Stir together water and corn syrup in a small saucepan; bring just to a boil over medium-high heat. With the food processor running, add the syrup mixture, then vanilla. Continue processing until the sauce is smooth, scraping down the sides as needed. (The sauce will seem thin, but will thicken during cooling.) Transfer to a container and refrigerate until chilled, at least 2 hours. Stir well before serving; if the sauce seems too thick, stir in a little water.
Make Ahead Tip: Cover and refrigerate for up to 2 weeks.