I have made this giant chocolate chip cookie cake many times, mainly for my daughter. She would rather have this than conventional cake on her birthday, so I am happy to oblige. It is very easy to make and you can change the topping as desired or leave off completely.
Preheat oven to 375 degrees. Spray a large round pizza pan OR a large cookie sheet with sides (mine measures 17.5 x 12 inches) with cooking spray; set aside.
Cream together the butter, brown sugar and granulated sugar (I used my stand mixer, but you can use a hand-held one). Add the eggs, one at a time and beat well after each addition.
In a separate bowl, put the flour, baking soda and salt and mix together (I use a wire whisk). Add the flour mixture to the creamed mixture mixing thoroughly to incorporate. Add the vanilla extract and mix again. Fold in the chocolate chips and the chopped nuts to combine.
Put the cookie dough on the prepared pizza pan or cookie sheet; using the back of a large spoon, spread the dough evenly on the pan. Sprinkle top of dough with M & M's...more or less to your own liking!
Bake in the center of the oven until edges are browned; the center will still look UNCOOKED but take it out anyway. The pizza size should take about 20 minutes (a little thicker than cookie sheet) and my large cookie sheet took only about 12 minutes.
NOTE: KEEP AN EYE ON IT SO YOU DON'T OVERCOOK IT... YOU WANT THIS NICE AND SOFT AND OOEY, GOOEY!!!
Set on cooling rack until almost completed cooled; cut pizza pan in a couple rows of wedges or cookie sheet into squares of size desired.