Preheat oven to 350F (for a glass pan), 325F (for a dark pan).
Line a 9 x 13 pan with parchment paper and set aside.
In a large bowl, crumble up the cookie dough and stir in oats, one cup M&Ms and peanut butter chips. Mix until combined.
Press the dough evenly into the prepared pan.
Bake for 10 minutes.
While dough is baking, combine sweetened condensed milk and chocolate chips in a microwave safe container. Heat on high heat for 30 seconds, stir. Repeat until smooth. Stir in vanilla extract.
Pour the fudge mixture over the partially baked crust and carefully spread. The dough is not fully baked so make sure to evenly pour the fudge to start with.
Sprinkle with additional mini M&Ms, the amount is up to you. I did about 1 cup.
Return to the oven and bake for another 15 to 18 minutes or just until the edges and center begin to crack.
Let bars cool completely on a wire rack before cutting into bars. I like to refrigerate my bars for a good hour so the fudge is nice and set up.