In a small saucepan, melt the butter. In one bowl, mix together the plain flour, cornflour and bicarbonate of soda. In another, mix together the soft dark brown sugar, egg and vanilla extract until combined and sugar has dissolved.
Pour the melted butter into the sugar bowl and combine. Fold in the flour mix and then the chocolate chips.
Go grab your cast-iron pan and spoon the mixture into it. Level it all out with your hands or the back of a spoon. Press it all down so it's flat. Bake in the oven at 200°c for 10 minutes (less if you like it to be really soft. It will be going back into the oven later, so consider this!)
Once the cookie has baked to your liking, remove from the oven and pour over the topping chocolate chips and marshmallows. Return to the oven for an extra 5 minutes, until the chocolate has melted and the marshmallows have toasted. Keep an eye on it as the marshmallows will turn from light to dark super quickly!
Serve straight away, either on its own or with ice cream.