Line either a baking tray (if you want small bite-size rocky roads) or one deep, small square baking tin (if you want to chill it whole and cut into slices) with foil or baking parchment.
Put the chocolate and butter in a microwavable dish and heat for 30 seconds. Give it a stir and put it back, for 20 seconds. Repeat the stirring and 20 seconds of heating until everything has melted.
Place the biscuits and mini marshmallows into a bowl and mix with the melted chocolate. Make sure that everything is coated. Transfer to the prepared baking tin depending on your preference.
Finally, put it in the fridge for 30 minutes until firm. If it isn't hard enough for you, pop it back in for about 15 minutes.
I like my rocky road not too hard, but some people like it hard. If you like it hard you can always put it back in the fridge for more time. Store in an airtight container.
Also, if you think your marshmallows are too big, you can always chop them up, not too small though!