Preheat the oven to 360℉/170℃. 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, cornflour and chocolate chunks. Tip into the lined tin and press to the edges (I do this with my hands as it's much easier!).
Bake for approx 25 minutes, until the top is just going gold. Remove and let cool before slicing and serving!