1. Add the soft butter, sugar, and hazelnut butter to a large bowl and mix on high for 2-3 minutes until very light and fluffy. Add the egg and vanilla extract and mix well. Mix flour, cocoa powder, cornstarch, baking powder, and salt, then add to the bowl and mix until just combined. Next, add the chopped chocolate and walnuts and fold in. Place the bowl with the dough in the fridge for at least 30 minutes.

2. Preheat the oven to 375°F (190°C). Line a baking sheet with baking parchment and set aside. Take the dough out of the fridge and divide it into eight or nine portions. Shape each dough portion into a ball. Break the balls apart to create two halves with a crumbly surface. Press these two pieces together, so the crumbly surface is facing up. Try not to create cookies that are too tall – they are not spreading a lot when baked so you want them to be already slightly flatter. If you like, press some extra chocolate into the dough.

3. Place the cookies on the prepared baking sheet and bake them for about 12 minutes. Remove the cookies from the oven and let them cool down on the baking sheet for some time, then transfer them to a wire rack and let them cool down completely.