Preheat your oven to 180ºc/160ºfan and lightly grease your skillet. My skillet is 23cm in diameter!
Whisk together the plain flour, bicarbonate of soda, salt and cornflour together in a bowl and leave to the side for now.
Melt the unsalted butter until liquid, and add to a bowl. Add in the light brown sugar and granulated sugar and whisk the mixture together for two minutes by an electric whisk, 3 minutes by hand.
Add in the egg and vanilla and whisk again until they're combined.
Add in the mixed together dry ingredients and mix until a really thick cookie dough is formed.
Add in the chocolate chips and beat in again.
Spread half of the cookie dough onto the bottom of the skillet.
Dollop/spread the caramel over the top of the cookie.
Add the second half of the cookie on top - I found it easiest to spoon small spoonfuls on top, as the caramel will make it hard to spread.
Bake the caramel skillet cookie in the oven for 20-25 minutes. It will still wobble somewhat, but you don't want it to be liquid.
Once baked, leave to cool for 10-20 minutes at least so you don't burn your mouth, and then serve!
Notes
This is best served fresh and warm.
As mentioned, I use an enamel covered skillet - but any sort of pie/dessert dish about 9-10" width is suitable.
If you don't finish it all, let it cool and just cover the top and leave at room temp.