The dough can be made the day before baking. In a large bowl, combine with butter and sugars. Add the egg and vanilla, followed by the dry ingredients. Add the chocolate chips or other additions last. Wrap with plastic wrap to chill in fridge overnight or for several hours, at least.
Preheat oven to 350 degrees. Add a tablespoon of butter to a 10-inch cast iron skillet and set it in the oven for a few minutes.
On the plastic wrap, form the dough into the rough shape of the skillet. When the butter has melted in the skillet, pull it out and rotate it to cover the base with butter. Carefully add the cookie dough, and using a spatula, press it in to the edges.
Bake until the cookie is golden brown and just set in the center, about 18 to 20 minutes. Let cool for 10 minutes, at least, or 30 minutes if you want it set enough to slice.
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