Preheat the oven to 180°C/360°F (without fan).
Cream the butter and sugar until the mixture is light and fluffy.
Beat in the vanilla and egg.
Add the flour, salt and baking soda.
Beat until you have a thick and smooth dough.
Stir through the chocolate chips.
Transfer the cookie dough to a cast-iron skillet. I use one which measures 24 cm/9.5 inches at the base, but please see my Kitchen Notes below.
Use a spatula to evenly spread the dough in the pan.
Place the skillet on the middle shelf in the oven.
Bake for 15-20 minutes, or until the cookie is lightly golden and the surface is still a little soft to the touch.
Serve warm with vanilla ice-cream.