Gently melt the marshmallows, butter and dark chocolate in a pan over a low heat, then leave to cool slightly. Crush about 10 biscuits into chunks and these to the chocolate together with the crumbs.
Whip the double cream until thick, then fold it into the chocolate mixture.
Spoon the mixture into a glass or ceramic dish and push some wine gums & sweets halfway into the mousse.
Scatter the sprinkles all over then position some wine gums on the surface. Chill in the fridge for 1 hour to set.
Decorate 5 biscuits with the writer to look like gravestones - write Boo, RIP or draw spider web and skeleton on them.
Just before serving, gently push the gravestone biscuits into the mousse and poke in a couple of fresh mint sprigs.