Preheat oven to 350 and grease a cast iron skillet.
In a large bowl, cream together butter, flour and eggs. Mix in Pillsbury™ Funfetti® Holiday Cake Mix until batter is smooth.
Stir in M&M's® and white chocolate chips by hand.
Using the back of a spoon, or your hands, press cookie dough into prepared skillet.
Bake for 25 minutes, or less if you like a gooey center.