Combine butter and sugars in a bowl with electric mixer on medium speed. Mix until light and fluffy. Mix in egg and vanilla. Stir in flour, baking soda and salt. Stir in chocolate chips.
Transfer dough to four 5-inch cast iron skillets or one 10-inch cast iron skillet. Spread evenly and smooth top.
Bake 18 - 20 minutes, a little longer for the 10-inch skillet, until golden brown and just set in the center. Cool 5 minutes before serving.
Notes
The center will fall a bit but the taste is not affected.