Prior to starting the cookie dough, line a small baking sheet or plate with parchment paper. Scoop out eight 1 tbsp. scoops of chocolate hazelnut spread on to the parchment paper, and set the sheet in the freezer to let the nutella freeze (this takes ~1 hr., you can do this the night before baking the cookies to make the process easier.)
Add the butter to a small saucepan set over medium heat. Stir the butter occasionally until it turns amber in color and develops little brown specks.
Transfer the brown butter to a medium sized mixing bowl and set it in the freezer for 5 minutes to chill. You should be left with 90g of brown butter, if you have a little less (2-3g), just add in some water until you reach 90g.
(OPTIONAL) In the meantime, you can toast up the hazelnuts so they'll have more flavor in the cookies. Add the hazelnuts to the saucepan you browned the butter in, and set it over medium heat. Stir occasionally until the hazelnuts are visibly golden brown. Remove them from the heat and set aside.
Once the butter has cooled down a bit, whisk in both sugars.
Whisk in the egg and vanilla until well combined.
Add in the flour, baking powder, baking soda, and salt. Use a rubber spatula to fold the mixture together just until a few flour streaks remain.
Add in the chocolate and crushed hazelnuts and fold the mixture together just until no flour streaks remain.
Scoop out heaping 3 tbsp. scoops of the cookie dough. Use your palm to flatten the cookie dough ball down, then place a frozen ball of chocolate hazelnut spread in the center of the flattened cookie.
Fold the overhanging cookie dough on top of the spread, making sure the nutella is completely trapped inside each cookie
Place the filled cookie dough balls back into the fridge for 30 minutes.**
Preheat the oven to 350°F / 175°C.
Evenly space out the cookie dough balls on a parchment lined cookie sheet. If desired, melt some extra chocolate hazelnut spread and use a spoon to top each cookie dough ball off with nutella.
Bake for 12-14 minutes, or until the edges of the cookies start to turn golden brown.
Once the cookies come out of the oven, I recommend "scooting" them. This just means take a bowl, cup, or biscuit cutter that is slightly larger than the cookie, place it on top of the cookie, and swirl the bowl around to make the cookie a little more round.
Let the cookies sit on the hot baking tray for at least 5 minutes before transferring them to a wire rack to cool. Serve and enjoy!