Line a baking tray with baking paper. Blow the balloons up so they're about as big as a grapefruit.
Place the chocolate into a large bowl and set it over a pan of simmering water on the stove. Stir until just melted then remove from the heat. Dip the base of the balloons into the chocolate, hold for a few seconds, then dip again and set on the lined tray. Place into the fridge.
Once the chocolate has set, pop the balloons and remove them so you have a chocolate cup. Leave in the fridge whilst you make the mousse.
Mousse
Place coffee and marshmallows into a medium pot and heat over a medium heat stirring until melted and sticky. Turn the heat down to low and take off the heat and set aside to cool.
In a large bowl, whip the double cream until you get soft peaks. Fold 3/4 of this into the marshmallow mixture, saving 1/3 for later.
Spoon your mixture into the chocolate cups. Top with a spoonful of your leftover whipped cream and shave some dark chocolate over the top using a vegetable peeler or paring knife.
Chill for at least 1-2 hours until set. Keep chilled until serving.
