To make the ganache, use a 2:1 ratio of cream to chocolate. Measure 200g of double cream into a pan with vanilla essence and heat until nearly boiling. Pour over broken up chocolate and stir until smooth and leave to cool.
To make the cocktail, add all ingredients into a shaker with cubed ice. Shake until thoroughly combine and cool. Double strain into a martini glass and garnish with a little grated chocolate.