Gently heat the double cream and butter until just steaming. Put the dark chocolate in a heatproof bowl, pour over the warm cream and stir to melt. Stir in the marsala, then mix the espresso powder with 1 tbsp boiling water and add this as well. Chill for 1-2 hrs, or until firm enough to scoop. Use a teaspoon to scoop the mixture and gently roll into balls using your hands (you may want to use gloves). Put on a lined baking tray and freeze for 30 mins.

Heat the white chocolate in the microwave for 1-2 mins, or until melted, then dip each ball in the white chocolate and return to the tray using a fork. Dust with a little espresso powder while the chocolate is still soft. Return to the freezer for 10-15 mins to set. Will keep in the fridge for three days.