1. Preheat the oven to 375°.
2. In a large bowl, using an electric mixer, beat both sugars and the butter until smooth. Beat in the vanilla. Add the eggs, one at a time, beating well after each addition. Add the flour, baking soda, and salt. Stir well with a wooden spoon. The dough will be stiff. Stir in the chocolate chips.
3. Spray a 10-inch cast iron skillet with cooking spray. Pat the cookie dough evenly into the skillet and decorate the surface with M&M's or sprinkles. Bake until the edges are lightly browned, about 30 minutes.
4. Remove the skillet from the oven and let cool on a wire rack for 15 minutes. Cut the cookie into 16 wedges and serve it directly from the skillet using a pie server.