Melt the marshmallows, butter and dark chocolate over a low heat, then leave to cool slightly. Crumble in 13 of the biscuits (making them quite chunky crumbs).
Whip the double cream until thick, then fold through the chocolate mixture. Spoon into a 1.5-litre dish and sprinkle with half a teaspoon of each of the Halloween sprinkles.
Position the mice and wine gums on the surface. Chill in the fridge for 1 hour to set.
Decorate 5 biscuits with the writing icing pens from the Halloween decorating kit to look like gravestones.
When ready to serve, arrange the gravestone biscuits in the mousse and poke in sprigs of fresh mint for a fun and frightening treat.