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.
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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.
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Add both egg yolks and vanilla extract to this bowl and beat on medium speed for around 20 seconds until combined.
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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.
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Add the dark and white chocolate chips to the bowl and beat on low until just combined.
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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.
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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.