In a medium bowl stir together the flour and baking soda.
Use your cast iron skillet to melt 9 tablespoons of butter over medium heat. Stir the butter consistently until it gets dark golden brown, about 5 minutes. It will have a nutty smell and the bubbling will go down.
Pour the browned butter into a large bowl. Then stir in the remaining 3 tablespoons of butter. Stir until totally melted.
When the skillet has cooled, wipe out any excess butter with paper towels.
Add the brown sugar, sugar, vanilla, and salt into the melted butter. Whisk until smooth.
Whisk in egg and yolk until smooth, about 30 minutes. Then let it sit for 3 minutes.
Repeat (two times) stirring for 30 seconds, then sitting for 3 minutes.
Fold the dry ingredients into the butter mixture.
Fold in the chocolate chips. Make sure there aren't any pockets of flour.
Press the cookie dough into the skillet. Use a spatula or your hand to press the dough evenly into the skillet. You can also press a few extra chocolate chips on top if you like.
Put the skillet in the oven and bake for 20-25 minutes. The cookie is done when it is golden and the edges are set and slightly darker.
Let skillet cool on a wire rack or 20 minutes.
Slice into wedges and serve, preferably with ice cream.
Recipe by Cooking is Messy at https://www.cookingismessy.com/2017/01/20/skillet-chocolate-chip-cookie/