Preheat the oven to 375 degrees Fahrenheit.
Using a stand mixer fitted with a paddle attachment; beat the room temperature butter, granulated white sugar and light brown sugar together for 1 minute on medium speed until creamy. Then add in the eggs, vanilla extract, and baking soda. Mix again for 30 seconds.
Add the all-purpose flour to the mixture, and mix on low for 20 seconds then medium speed for 40 seconds until well incorporated.
Stop the mixer, add in the chocolate chips, chopped chocolate Halloween candy, M&M's and 2 tablespoons of the sprinkles. Mix on low for 10-15 seconds until just incorporated.
Grease a 9-inch cast-iron skillet with vegetable oil. Wipe it clean so it's just greased but not puddling. Then scoop the cookie batter into the skillet. Flatten with a silicone spatula and sprinkle the remaining 1 tablespoon of sprinkles on top of the cookie.
Bake for 30 minutes until golden brown. Carefully, remove the skillet from the oven and let rest in skillet for 30 minutes. Cut into wedges, and serve.