Start by adding the spiced rum, Fernet, and crème de cacao to your shaker.
Next, add the black food colouring. Give the ingredients a hard, dry shake (no ice). This will help thoroughly incorporate the ink.
Fill your shaker with ice and shake vigorously for a full 30 seconds so that your drink becomes cold and frothy.
Double strain into a chilled cocktail glass and top with the chocolate stout.