Preheat your oven to 350°F (175°C). Line your 9x13 inch baking pan with parchment paper, leaving some overhang on the sides for easy removal later.
In a large mixing bowl, combine the semi-sweet chocolate chips and butterscotch chips. Microwave them in 30-second intervals, stirring in between, until completely melted and smooth. Be careful not to overheat!
Once the chocolate and butterscotch mixture is melted, stir in the creamy peanut butter until fully incorporated. The mixture should be smooth and glossy.
Add the mini marshmallows and M&Ms to the chocolate-peanut butter mixture. Gently fold them in until they are evenly distributed throughout.
Pour the mixture into the prepared baking pan, spreading it out evenly with a spatula. Make sure it reaches all corners of the pan.
Bake in the preheated oven for 20-25 minutes, or until the edges are set and the center is slightly soft. The bars will firm up as they cool.
Remove the pan from the oven and let it cool completely on a wire rack. Once cooled, lift the bars out of the pan using the parchment paper overhang. Cut into squares and serve!
