Stir, stain, straight up, cocktail glass, garnish
The original called for Bittermen's Sweet Chocolate bitters, a one-off bitters made by a measuring error when making their Mole bitters.
This recipe previously listed a variation using rye, regular sweet vermouth, and omitting the bitters. Corrected thanks to a comment by Campus Five.
Dark Spirits, A. J. Rathbun, Melissa Punch