Line your baking tin with cling wrap or baking paper
Melt the marshmallows and butter in a heatproof bowl in the microwave, stirring every 30 seconds. (Mine only took 1 minute in my microwave but it depends on each one). If you don't have a microwave you can do this in a small saucepan on medium heat, stirring continuously.
After you have melted the marshmallow and butter mixture, stir in the rice bubbles. Once they are fully combined, press into the tin, making sure you get right to the edges. Make it as flat and smooth as possible.
Refrigerate for 2 hours, then remove from the tin and slice into bars.
Store in the fridge in an airtight container.