Preheat oven 190C/170C fan. Grease & line 9x9inch tray
Whisk flour, bicarbonate of soda, salt & cornflour in a bowl & leave to the side
Melt the unsalted butter to liquid, add in the light brown sugar & granulated sugar, whisk together for 2 mins with electric whisk & 3 mins by hand
Add in the egg & vanilla, whisk until combined
Add the dry ingredients, whisk until a thick cookie dough is formed
Mix the chopped chocolate into the dough (reserve some chocolate to decorate)
Push the mixture into the tin & add reserved chocolate to decorate
Bake for 17-22 minutes
Allow to cool in the tin for 10 minutes, then remove to cool completely before cutting