Melt 9 tablespoons butter in your cast iron skillet over medium heat.
Cook, stirring constantly until the butter is dark golden brown. Should take around 5 minutes. Pour butter into a large bowl. Add the 3 tablespoons of butter to the hot butter.
Add the sugar, brown sugar, vanilla and salt with the butter. Stir until smooth.
Add the egg and yolk and whisk until combined and nice and smooth.
In a separate bowl add flour and 1/2 teaspoon baking soda. Whisk to combine.
Add flour mixture to the butter mixture and stir to combine.
Add the chocolate chips and mix together.
Wipe cast iron skillet clean
Transfer the dough to your cast iron skillet. Press into an even layer.