Line a 9x5 inch loaf pan with parchment paper or waxed paper.
Set a heatproof bowl over a pan of barely simmering water. Add the chocolate chips and butter and stir until melted and smooth. Remove from over the heat and let cool to lukewarm.
Fold in the crackers and marshmallows until well coated in the chocolate. Transfer to the prepared pan and spread as evenly as possible.
Refrigerate 1 hour, until firm. Then lift the bars out by the parchment and use a large sharp knife to cut into 24 mini bites or 12 larger bars.
