In a large bowl, combine butter with peanut butter and powdered sugar and mix until it is fully combined.
Fold in M&Ms. Press the peanut butter dough into a square baking dish. Refrigerate for at least 20 minutes.
Place the chocolate chips into a microwave-safe bowl, and heat in the microwave in 30-second intervals, stirring after each until it is fully melted and smooth.
Evenly pour the melted chocolate over the peanut butter bars. Refrigerate for another 20 minutes. Cut into bars.