Preheat oven to 350 degrees F.
Spray non stick spray or lightly grease your cast iron pan and set aside.
With the aide of a mixer, blend butter and sugars together until smooth.
Turn off mixer.
Add the egg and vanilla until mixed all the way through and the mixture is golden brown.
Gently add flour, baking soda, and salt together.
Mix together until blended but mix only a few times as to not make the dough dense.
With a rubber spatula fold in the chocolate chips and m&m's.
Spoon in the cookie dough unto the cast iron pan.
Press with your fingers around to flatten into the pan and cover the pan completely with the cookie dough.
Place in the oven for 17-20 minutes. Checking often.
Take out of the oven once you have checked the center is done and let it rest until the cookie is almost cool and serve with a scoop of ice cream.