Line a 9 x 13 inch pan with baking paper.
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.
*Note about measuring the Cheerios: If you're using UK cups (slightly larger than US cups), use just 5 cups of Cheerios (about 240 - 250 grams). Don't worry about being super precise - it won't ruin the recipe!
Remember to melt your butter, peanut butter and marshmallows ON A LOW HEAT. You don’t want it to boil up into a super chewy caramel!
If you like, you can replace ½ cup of the Cheerios with ½ cup peanuts (I use unsalted but salted may work well too!).
These peanut butter Cheerio treats stay crispy for a day or two in an airtight container, but not too much longer. If you want to keep them longer, store in the freezer in Ziploc bags.