Add all the ingredients - except the cream and marshmallow - to a cocktail shaker, add plenty of ice and shake vigorously until chilled. Strain into a chilled martini glass. Carefully pour the cream over the back of a bar spoon to layer on top. Garnish with the toasted marshmallow.
Add all the ingredients to your Tesco, Ocado etc. baskets.
This drink can be made the more traditional way by combining the cream with the other ingredients in the cocktail shaker, instead of layering over the top as shown here. 12.5ml of fresh espresso coffee can be used in place of the concentrate, though be sure to add more ice to the shaker.