Put the chocolate in a glass bowl. Put the bowl in a steamer and melt over simmering water ( make sure the bottom of the glass bowl doesn’t touch the water).
When the chocolate has melted, take it the heat and add ⅔ Tbsp of double cream and mix well (add some marshmallows now, if you like a gooey version). Let it cool a bit.
Meanwhile, whip the cream (and sugar, if used) until soft peaks form - it should be fluffy, must not be stiff!
Gently fold the chocolate in the cream.
Spoon the mousse into the glasses. Cover the top with cocoa powder, chocolate sprinkles or shavings.
Using the chocolate pen, write RIP on 4-6 cookies - push these gently into the mousse, and dust the “graveyard” with cocoa powder.
Serve immediately or chill until needed.