Start by greasing a 9 x 13-inch baking pan with non-stick spray to ensure the Peanut Butter Cheerio Bars won't stick.

In a large saucepan, melt 1/4 cup of unsalted butter over low heat. Stir gently until liquefied, about 2-3 minutes. Add in 1 cup of smooth peanut butter and 4 cups of mini marshmallows, continuously stirring to create a creamy mixture.

Continue stirring on low heat until all ingredients are fully melted and combined, about 3-5 minutes.

Remove from heat and gently fold in 4 cups of Cheerios, ensuring every piece is coated with the mixture.

Pour the mixture into the prepared baking pan and spread it evenly. Press it down firmly to compact it, and allow to cool at room temperature for about 1 hour until set.