Beat the unsalted butter until fluffy and pale.
Then sift in the icing sugar a bit at time until fully incorporated then sift in the cocoa powder.
if the mixture is a little bit stiff then add some milk until the mixture is soft. Then once the mixture is ready, fill a piping bag with a round nozzle.
Pipe some buttercream on to the top of the cupcakes then top with a digestive biscuit the pipe more buttercream on to the biscuit and top with mini marshmallows.
Toast the marshmallows and your ready to enjoy!