In a medium-sized container, mix the melted butter, ground Cheerios™, powdered sugar and peanut butter until all ingredients are fully incorporated, forming a kind of creamy batter. Add 1 tablespoon of the Cheerios™ batter to each individual mold in a mini muffin pan, filling or preparing 21 mini cupcakes. Melt the chocolate in the microwave. Pour 1 tablespoon of the melted chocolate over the layer of Cheerios™ batter. Refrigerate for 3 hours, or until the cupcakes have set and are completely firm. Store in the refrigerator. Serve and enjoy!