Preheat oven to 350 degrees. Line with parchment or spray with non-stick cooking spray 2 large sheet pans. Set aside.
In a large mixing bowl, combine dry cake mix, brown sugar, egg, milk, oats, peanut butter, chocolate chips, m&ms, and chopped nuts until well mixed.
Using a large 2 inch cookie scoop, place mounds of cookie dough on prepared sheet pans. Using a small piece of parchment paper and the bottom of a glass, flatten out the cookie dough leaving 1/4 inch space between each flattened cookie dough.
Bake in preheated oven for 14-16 minutes until crisp on the edges, golden brown on bottom and set on the top. NOTE: (Cookies will be cake like in the center.) Pull sheet pans out of oven and sprinkle extra m&ms on top of each cookie while hot and push them into the cookie. Allow cookies to rest on sheet pans for 10 minutes or so before removing to a wire rack to finish cooling.