Preheat the oven to 350 degrees Fahrenheit or 175 degrees Celsius.

Take a 10-inch oven-proof cast-iron skillet and grease it with butter or non-stick spray.

In a mixing bowl, add all-purpose flour, baking soda, and salt; mix to combine, then set aside.

Add butter, sugar, and brown sugar into a bowl and mix everything with a whisk.

Now add an egg, an egg yolk, and vanilla extract, then mix again until well combined.

Add the flour mixture to the egg mixture and mix until just well combined.

Now, add the chocolate chips and fold them into the dough.

Now, take the skillet and press and spread the cookie dough into it.

Put the skillet in the preheated oven and bake for 20 to 25 minutes, until the cookie turns golden brown and the centre moves slightly but doesn't look wet. (It will firm up as it cools down)

Cool the cookie for about 15-20 minutes at room temperature.

Then, serve with any kind of topping you want, like vanilla ice cream, chocolate sauce, whipped cream, etc.