In a heatproof bowl, melt the chocolate chips, peanut butter, and butter together. You can do this in the microwave in 20-second intervals or using a double boiler on the stovetop. Stir until smooth.

Once the mixture is smooth, remove it from the heat and let it cool slightly.

While the chocolate mixture is cooling, line a square baking dish with parchment paper.

Once the mixture has cooled a bit, fold in the marshmallows, ensuring they are evenly distributed.

Transfer the mixture into the prepared baking dish and spread it out evenly.

Place the baking dish in the refrigerator and let it set for at least 2 hours.

Once the bars have set, remove them from the fridge and cut them into 12 squares.

Serve and enjoy your delicious simple Chocolate Marshmallow Bars!