1. Add both sugars and the cold butter in small pieces to the bowl of a stand mixer. Mix on low until the butter and sugar have merged into streusels – you don’t want the sugar and butter to fully combine. Add the chopped chocolate and mix it in briefly. Combine the flour with baking powder and salt. Add the dry ingredients to the bowl and mix to combine. You want the mixture (once again) to look like streusels instead of a proper cookie dough. Next, add the chopped walnuts and mix them in briefly.

2. Lightly whisk the egg with the vanilla extract in a small bowl, then add this to the bowl of the stand mixer and mix everything until the streusels start sticking together. You want a very rough-looking dough. Divide the dough into six portions and shape them into balls. Transfer the balls to a freezer-safe container or freezer bag and place them in the freezer overnight.

3. Preheat the oven to 180°C (350°F) fan-forced. Line a baking sheet with baking parchment. Place the frozen dough balls with enough space in between on the baking sheet and bake the cookies for about 18-19 minutes – they should look nicely browned all around. Take them out of the oven and let them cool down on the baking sheet for a moment. If the cookies look a bit deformed, bring them into shape right after baking. Transfer the cookies to a wire rack and let them cool down completely.