Start the day before you want to bake the cookies.
Whisk together the olive oil, light brown soft sugar, caster/granulated sugar, milk and vanilla extract in a large bowl until very well combined.
In a separate bowl sift together the plain flour, bread flour, cornflour, baking powder, bicarbonate of soda and salt then stir in the chopped chocolate.
Tip the dry ingredients into the wet and stir until no dry patches remain but don't over-beat the dough. The dough will be a bit soft at this point but will firm up as it chills.
Cover the bowl and place in the fridge to chill overnight. The dough will keep in the fridge for up to two days.
The following day preheat the oven to 180°C/160°C fan/350°F/gas mark 4. Line a couple of metal baking sheets with baking parchment.
Use a cookie scoop or tablespoon measure to scoop out balls of dough - I do a heaped tablespoon per cookie. Or just roll it into balls with your hands.
Place the balls of dough well spaced apart on the baking sheets. Bake the cookies for 10-15 minutes until they are golden around the edges.
When you remove the trays from the oven, bash them on the countertop a couple of times; this will flatten out the cookies and give them the classic cookie appearance.
Let the cookies cool on the tray for ten minutes then transfer them to a wire rack to cool completely. Once cold, store in an airtight container.