Cocoa beans and coffee beans have similar fruity, spicy flavor elements and by adding coffee to chocolatey recipes, you deepen and enhance the rich, chocolatey flavor of the chocolate. In small amounts the coffee does not add a discernible coffee flavor, it simply makes the chocolate flavor more intense, in a similar way to how salt works when added to food. When added in the right quantity, you don't actually taste the salt in a recipe, but you know it's there because it enhances the flavor and makes the food taste better. It's a tip that's well worth remembering when making anything chocolatey like chocolate cake, cookies etc.