If needed, chop the dark chocolate into chunks. Add the chocolate to a heatproof bowl and microwave for 30-second intervals, stirring in between until fully melted. Scoop into an 8-ounce glass.
Pour freshly-brewed coffee overtop, then add the demerara sugar and Irish whiskey and stir for 30 seconds.
In a measuring cup with a spout, froth the heavy cream using the milk frother until thickened, 15-20 seconds. Hover a teaspoon with the underside facing up over the coffee mixture and pour the cream overtop.
Garnish as desired with either ground nutmeg with an espresso bean, or simply use chocolate shavings and serve immediately.
