In a bowl of a stand mixer, using the paddle attachment, or using a hand mixer, cream the butter, granulated sugar and brown sugar for 3 minutes, so it becomes light and fluffy.
150 g butter, 50 g granulated sugar, 150 g dark brown sugar
Add the egg, egg yolk and vanilla extract and mix until just combined.
1 large egg, 1 egg yolk, 1 tsp vanilla extract
Then add the flour, cornstarch, baking powder, baking soda and salt to the bowl and mix on low speed until the flour is halfway mixed in.
320 g all-purpose flour, 2 tbsp cornstarch, 1 ½ tsp baking powder, ½ tsp baking soda, ½ tsp salt
Then chop the milk and dark chocolate into smaller pieces and add it to the dough. Mix the dough together until everything has just combined. You don’t want to overmix the batter.
125 g dark chocolate, 125 g milk chocolate
Divide the dough into 6 cookie dough balls of 165 g (5.8 oz) each and roughly shape them into a ball. Note: If you want a rugged looking cookie, try not to roll the cookie dough ball too smooth. This is purely an aesthetic thing and doesn’t affect the final result in any way.
Let them rest in the fridge for 1 hour. While they rest, preheat the oven at 200ºC / 395ºF and prepare a baking tray with baking paper.
Place the 6 cookies on the baking tray and bake them for 13-14 minutes or until you can see some of the edges in the bottom start to become golden, for a soft and chewy cookie.
Once baked, let them cool down on the baking tray for 5 minutes as they still will be soft when they are done. After they have cooled down a bit, use a spatula to lift them onto a cooling rack. Enjoy them warm or cooled down.