1. In a large saucepan, melt butter over low heat. Add marshmallows and stir until completely melted. Remove from heat. Stir in peanut butter until melted.
2. Add Cheerios cereal. Stir until well-coated.
3. Evenly press the mixture into a 13 x 9 x 2-inch pan coated with cooking spray. (Be careful, it can be very warm!)
4. Add your chocolate chips and shortening or oil to a heatproof bowl and microwave for 30 seconds. Stir, then microwave for an additional 20-25 seconds before stirring again. Continue heating in 15-second intervals, stirring in between, until melted and smooth.
5. Drizzle chocolate over cereal bars.
6. Cool. Cut into 2-inch squares.

Enjoy!