Measure 200ml gin or vodka in a jug. Melt 50g dark chocolate (70% cocoa solids) and pour into the alcohol, whisking all the time, until smoothly blended. Add 100ml chocolate liqueur, such as black mozart (from specialist off-licences).
For each cocktail, shake 100ml of the chocolate liquid with crushed ice and strain into a martini glass. Decorate with a curl of lemon zest.