Add the butter, cocoa powder and mini marshmallows to a saucepan. Place over low heat and stir gently until melted.
Remove the pan from the heat and stir in the Rice Krispies.
Press the mixture into your lined tin with a metal spoon or silicone spatula. If the mixture is too sticky, wait a couple of minutes before trying again.
Melt the chocolate and butter together over low heat then spread it over the Rice Krispie traybake.
Pop it in the fridge for 2 hours to set, then remove it from the tin, cut it into bars and enjoy.
Notes
Store in an airtight container in the fridge for up to 1 week.