In a medium bowl, whisk together the coffee, sugar and cocoa. Whisk in the corn syrup until all the sugar crystals are dissolved and the mixture is completely smooth.
Place the chocolate and butter in a large, dry bowl or in the top of a double boiler set over barely simmering water. When the chocolate has melted, whisk in the coffee-cocoa mixture until smooth and blended. The sauce is best if left to ripen for a full day before serving. Store in a sealed container. The sauce keeps for 3 weeks in the refrigerator or 3 months in the freezer.