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 melted and smooth, 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.