Preheat the oven to 200C/180C Fan/Gas 6. Line an 8′ Square Baking Pan (9″ will work for thinner bars) with baking parchment.
In a large bowl, or the bowl of a stand mixer, combine the butter, brown sugar and granulated sugar (medium speed) for about 3-5 minutes until fully combined.
Add the egg and vanilla and mix again.
Fold in the flour, baking soda and cornflour and fold together. Then gently fold in the milk chocolate.
Press half of the dough into the lined tin. This may require your hands as it is pretty firm. Place down the KitKat bars, side by side, without chopping them up. Leave some space at each edge. Cover with the remaining cookie dough.
Bake for 25-30 minutes, until entirely baked and just golden on top (don't worry if it looks a little underbaked - that keeps them chewy). If you have any remaining KitKats, chop them up and scatter on top!
Leave to cool on a wire rack for at least 45 minutes before attempting to cut (they will fall apart if cut any earlier).