Preheat your oven to 350°F (175°C). This ensures that your bars bake evenly.
Line a 9x13-inch baking dish with parchment paper, leaving some overhang on the sides for easy removal later.
In a large mixing bowl, combine the refrigerated chocolate chip cookie dough and oats. Mix until well incorporated.
Spread the cookie dough mixture evenly into the prepared baking dish. Use a spatula to press it down firmly.
In a separate bowl, combine the mini M&M's and Reese's peanut butter chips. Sprinkle this mixture evenly over the cookie dough layer.
In a microwave-safe bowl, combine the sweetened condensed milk and semi-sweet chocolate chips. Microwave in 30-second intervals, stirring in between, until the chocolate is melted and the mixture is smooth. Stir in the vanilla extract.
Pour the chocolate fudge mixture over the cookie dough and mix-ins. Use a spatula to spread it evenly.
Add a handful of mini M&M's on top of the fudge layer for that extra crunch and color.
Bake in the preheated oven for 25-30 minutes, or until the edges are golden brown and the center is set.
Once baked, remove the dish from the oven and let it cool completely in the pan. Use the parchment overhang to lift the bars out and cut into squares.