Preheat the oven to 350 degrees.
Spray a 9in cake pan with non stick spray. Set aside.
Cream together your butter and your white sugar in a large bowl.
Add in the egg, egg yolk, and the vanilla and stir in.
In another bowl mix together your flour, baking soda, cream of tartar, and salt.
Add your dry ingredients to your wet ingredients and stir everything together till just combined.
Add in your m&ms and white chocolate chips and fold into the batter. **
Press the cookie dough into the prepared cake pan.
Bake in preheated oven for about 25 minutes until the cookie is golden brown. (You can stick a toothpick in to make sure its not gooey in the middle)
Allow to cool for in the pan. Loosen the sides with a knife and put cake onto a platter or serving dish.