Line a 9 x 13 inch pan with baking paper.
Melt the butter, peanut butter and marshmallows together slowly on a low heat.
Combine with a wooden spoon, then stir in the Cheerios.
Press the mixture into the pan with damp hands to stop the mixture sticking.
Melt the chocolate in the microwave for 90 seconds, then pour over the Cheerio mixture, spreading right to the edges.
Let the Cheerio bars set in a cool place for a while (the fridge is fine), then cut into bars. Store in an airtight container for up to 2 days, or freeze in Ziploc bags.
