Preheat the oven to 180C/350F and spray a 10 or 12 inch cast iron skillet with cooking spray. Leave to one side
Place butterscotch chips, white chocolate chips, flour, cornflour, bicarb and salt into a medium sized bowl and stir to coat all the chips in flour. Leave to one side.
Place sugars and butter into a large bowl, or the bowl of your stand mixer and beat on medium-high speed until light and fluffy, around 2 minutes.
Add in the eggs and vanilla extract, scraping down the sides as necessary, and mix until well incorporated and smooth, around 1 minute on medium speed.
Tip in the flour mixture and mix on low until a soft dough forms and there are no lumps of flour.
Tip the dough into your prepared skillet and smooth with a spatula until you have an even layer of dough.
Place in the oven for 23-28 minutes, until slightly crisp around the edges, golden and risen all over and the top looks matte in the light of the oven. An inserted skewer into the middle will come out with a few crumbs. You still want it to be soft in the middle and slightly underdone, but it will still look matte on top.
Leave to cool for 30 minutes, before topping with vanilla ice cream and drizzling butterscotch sauce over the top. Grab a spoon and dig in!
Butterscotch White Chocolate Skillet Cookie will keep in an airtight container, at room temperature, for 5 days. I would advise taking it out of the skillet as it begins to taste metallic after a day in there!