Grease a 9 x 13-inch baking pan with non-stick spray.

In a large saucepan, melt ¼ cup of unsalted butter over low heat, stirring gently for about 2 minutes until liquid.

Add 2 cups of mini marshmallows and 1 cup of creamy peanut butter, stirring for another 5 minutes until smooth and glossy.

Remove from heat and pour in 4 cups of Cheerios, mixing thoroughly until every Cheerio is coated.

Transfer the mixture into the prepared baking pan and press down gently to fill it completely.

Let the bars cool at room temperature for at least 1 hour.

Once set, slice into squares and serve.