Preheat oven to 180°/160° fan.
If you're not using a silicon baking tray, line your baking tray with baking/parchment paper.
In a medium bowl whisk the plain flour, cocoa powder, cornflour, bicarbonate of soda and salt until combined. Set aside.
Using the bowl of your stand mixer (fitted with paddle attachment) or a large bowl if using an electric hand whisk, beat the butter and both sugars on medium speed for 1 whole minute until light and fluffy.
Add both egg yolks and vanilla extract to this bowl and beat on medium speed for around 20 seconds until combined.
Add half the dry ingredients to this bowl and beat on low speed until just combined. Add the remaining half and beat again on low speed until just combined.
Add the dark and white chocolate chips to the bowl and beat on low until just combined.
Measure out two level tablespoons of dough per cookie - press the two tablespoons of dough together, roll into a ball in your hands and place on your baking tray. I bake 6-7 cookies per tray to allow space for the cookies to spread slightly.
Bake the cookies for 10 minutes, turning the tray halfway through if your oven has a hot spot so that they bake evenly.
Once out of the oven, place the baking tray on your countertop and leave the cookies on the tray for a further 5 minutes - they will continue baking on the hot tray.
While they're still warm, use the remaining chocolate chips from your two bags of chips to press 3-4 gently onto each cookie. This is mainly for appearance but I think it makes such a difference!
After 5 minutes, remove the cookies from the baking tray using a spatula and place on a wire cooling rack. Do this carefully as the cookies will still be slightly soft.
Once completely cool, store in an airtight container at room temperature.
Best eaten within 2-3 days. They freeze really well too! Individually wrap each cookie so you can grab one as a quick snack.