Line a cookie tray with parchment paper. Using teaspoons, drop small dollops of Nutella onto the tray, making sure they are evenly spaced so they don’t stick to each other. Place the tray in the freezer and freeze for about 3 hours or overnight (overnight is better so do this the day before you make the cookies). If you’d like to skip this step, you can substitute the Nutella centers with a regular chocolate bar you love, just roughly chop it.
Toast the hazelnuts and set aside. In a bowl, add the all-purpose flour, baking soda, salt, and cardamom powder. Whisk well and set aside.
Melt half of the unsalted butter in a small pan over medium heat until it turns deep amber to create brown butter. Be careful not to walk away, as the butter can go from deep amber to burnt in seconds. As soon as the milk solids at the bottom of the pan have browned, remove the pan from the heat.
Using a stand mixer with the paddle attachment (or a hand mixer), cream the remaining unsalted butter with the fine white sugar until it becomes creamy and fluffy. Occasionally, pause to scrape the bottom and sides with the spatula to ensure everything is mixed together well. Then add the brown butter and brown sugar, and beat well for 4 minutes. A little science here: You might be wondering, ‘Why not just add everything and cream everything together?’ The reason being is because white sugar and dark brown sugar behave differently and are used for different purposes. The initial stage of beating white sugar with room temperature butter is to aerate the batter and give it volume and stability for the cookies. The addition of dark brown sugar and brown butter is to give it the nutty flavour and give the cookies a chewy and tender texture. Switch to the whisk attachment and add the egg (if making a larger batch, add one egg at a time), and vanilla extract. Whisk until everything is well incorporated.
Switch back to the paddle attachment, add the flour mixture, and mix on very low speed until you can barely see any flour in the dough. Add the toasted hazelnuts, chopped chocolate, and pretzels. Using a spatula, fold everything together until evenly combined.
With an ice cream scoop or a spoon, form the dough into 6 cookie dough balls and place them on a lined baking tray. Cover with plastic wrap and freeze for 30 minutes.
Preheat the oven to 170°C (340°F). Before baking, make sure the cookies are spaced well apart on the baking tray to prevent them from sticking together. If your baking tray is small, bake in batches and keep the remaining dough balls in the freezer.
Bake the cookies for 15–17 minutes.Round cookie tip: This method works best when the cookies are fresh out of the oven. Using a round cookie cutter that is larger than your cookies, place it over the cookie as soon as it gets out of the oven and gently swirl it around. As the cookie hits the edges of the cutter, it will start to take a more round shape because it is soft and hot enough to be moulded. Then sprinkle each cookie with a pinch of flaky sea salt (optional, but the saltiness balances the sweetness). Allow the cookies to rest for 10 minutes. If you love warm cookies, dig in! If you plan to store them or eat them later, use a flat spatula to transfer the cookies onto a wire rack to cool completely.