Preheat oven to 180°C (350°F) and grease a 9"x13" tin.
Crush the biscuits and melt the butter.
Mix together and spread over the base of the slice tin. Bake for 10 minutes.
Mix the cocoa and condensed milk together.
Pour over the slightly cooled biscuit base.
Sprinkle the chocolate chips on the condensed milk mixture (optional).
Sprinkle the marshmallows over top and return to oven for a further 20 minutes or until the marshmallows are nice a toasted.
Notes
In the tutorial I did, I heated the condensed milk and cocoa together. If you do this, it is a lot runnier and pierces the biscuit base so you have a gooey mess instead of a layered slice. It still tastes great though!