1. Preheat the oven to 180°C (350°F). Line a baking sheet with baking parchment and spread the walnuts evenly on top. Toast the nuts for 6-8 minutes until fragrant. Take them out and let them cool down completely. Chop coarsely and set aside.

2. Add the butter and sugars to a large bowl and mix until light and fluffy. Add the egg and vanilla extract and mix until well combined. Mix the flour with cornstarch, baking soda, and salt and add to the bowl – mix until just combined. Add the chocolate chips and chopped walnuts and fold in. Cover the bowl and place it in the fridge overnight.

3. Preheat the oven to 180°C (350°F). Line a baking sheet with baking parchment. Divide the dough into 10 equal portions and place five of them with enough space in between on the baking sheet – keep the rest in the fridge. Bake the first batch of cookies for about 12 minutes until lightly browned. They will look quite underbaked, but that is ok. Take them out of the oven and let them cool down on the baking sheet for about 10 minutes, then transfer them to a wire rack to cool down. Repeat with the second batch of dough.