Line a baking tray and break the chocolate into a heatproof bowl.

Add the butter into the chocolate and then place in the microwave in 20 second increments, stirring between each until the chocolate is gloriously smooth. Make sure you stir for about 30 seconds between heating as chocolate can hold its shape and be melted and then you'd end up with a burnt up mess. Alternatively, you can place the bowl on a simmering saucepan of water and stir until melted.

Pour the chocolate onto your baking tray and spread with a small offset spatula. My square ended up being 25cm x 28cm - this was a great thickness for the bark, but if you want it thicker then spread it out less.

Sprinkle the M&M's all over the melted chocolate.

Leave to set for at least 6 hours, overnight if possible. Once the chocolate is set break it into pieces.

Bark will keep in an airtight container, at room temperature, for 3 days.