Line an 8inch/20cm tin with baking parchment.
Melt the butter and mix together with the finely crushed Graham Crackers or Digestives. Press down firmly into your traybake tin.
Place the tin in the fridge (or even the freezer) for about 30 minutes to start to set.
To make the chocolate ganache layer, chop the chocolate into small pieces and place in a bowl. Heat the cream slowly in a small saucepan until almost boiling. Then pour the cream over the chocolate and whisk until smooth and all the chocolate has melted. Pour the chocolate mixture over the base and spread to the edges.
While the chocolate is still soft, place the mini marshmallows in a single layer over the top, trying to cover as much of the chocolate as possible
Place in the fridge for an hour or two to let the chocolate set. Then toast the marshmallows under a hot grill/broiler for 1-2 minutes until puffy and golden. Watch this closely because they do brown quickly.
At this stage the marshmallows are toasty and crispy, but if you leave the traybake for a few hours, the marshmallows become soft and gooey and sticky. And the chocolate becomes soft and gooey. Cut into bars and enjoy!