Preheat your oven to 350°F (175°C). This ensures that your Monster Fudge Bars bake evenly and come out perfectly gooey.
Line your 9x13 inch baking pan with parchment paper, leaving some overhang on the sides. This will make it easier to lift the bars out once they are cool.
In a mixing bowl, take the 2 cups of refrigerated chocolate chip cookie dough and spread it evenly across the bottom of the prepared pan. Use your fingers or a spatula to press it down firmly.
In a separate bowl, combine the oats and 1 cup of mini M&M's. Sprinkle this mixture over the cookie dough layer, distributing it evenly.
In a microwave-safe bowl, combine the 1 cup of Reese's peanut butter chips with the 14 oz of sweetened condensed milk. Microwave in 30-second intervals, stirring in between, until smooth and well combined.
Pour the melted peanut butter mixture over the oats and M&M's layer, spreading it out evenly with a spatula.
Sprinkle the 12 oz of semisweet chocolate chips over the peanut butter layer, ensuring they are evenly distributed.
Place the pan in the preheated oven and bake for 25-30 minutes, or until the edges are golden brown and the center is set. Be careful not to overbake!
Once baked, remove the pan from the oven and let it cool at room temperature for about 30 minutes. Then, refrigerate for at least 2 hours to allow the bars to set completely.
Once chilled, lift the bars out of the pan using the parchment paper overhang. Cut them into squares or rectangles and serve!