Ice cream, hot fudge sauce, and more candy bars, optional for serving
Place the chopped candy bars in the freezer while you prepare the cookie dough. This will ensure that they don't fall apart when you stir them into the dough.
Preheat oven to 350°F. In a large bowl using a hand mixer or stand mixer, beat butter and sugars for 1-2 minutes, until light and creamy. Add eggs and vanilla extract and beat to combine, scraping the sides of the bowl as needed.
In a separate bowl combine flour, baking soda, and salt. Add dry ingredients to the butter mixture and stir with a rubber spatula or wooden spoon until completely combined. Stir in half of the candy bars (from the freezer).
Scoop the dough into a well greased 10-inch cast iron skillet. Spread the dough into an even layer. Sprinkle the remaining candy bars on top. Bake for 25-30 minutes, until the center is just set and no longer jiggles; the edges should be golden brown. Be careful not to overbake. Allow to cool just slightly. Serve with ice cream, fudge sauce, and additional chopped candy bars.
This cookie is best served warm. Adjust cooking time based on the size of your skillet.