Preheat oven to 400° F and line 3 baking sheets with parchment paper. Set them aside for later.
Slice the cold butter into ¼ inch cubes and place in the standing mixer, along with the brown and cane sugars. Gradually increase speed and mix on high for 3-4 minutes until creamy. Scrape down the sides of the bowl with a rubber spatula halfway through.
1 cup cold, unsalted butter, 1 ¼ cup light brown sugar, ½ cup cane sugar
In a separate bowl, whisk together the cocoa powder, flour, sea salt, baking soda, and cornstarch.
⅔ cup Dutch-process cocoa powder*, 2 ¼ cup all purpose flour, 1 teaspoon fine sea salt, 1 teaspoon baking soda, 2 teaspoon cornstarch
After creaming the butter and sugar, add the eggs and vanilla extract. Mix on medium speed until smooth and creamy. Scrape down the sides of the bowl halfway through.
1 tablespoon vanilla extract, 2 large eggs
Add mixed dry ingredients to the standing mixer and mix on the lowest setting until just combined. Add chocolate chips while the dry ingredients are still mixing in--this minimizes the risk of overworking the cookie dough.
1 ½ cup milk chocolate chips
Roll the dough into 15 balls and place 5 on each baking sheet. Press down gently on each cookie to flatten them slightly.
Bake each cookie sheet separately for 8-10 minutes, or until the tops start cracking. Mine are usually done right at 9 minutes. Immediately out of the oven, round out the edges of the cookies with either a round cookie cutter or the back of a spoon (optional).Allow the cookies to cool on the pan for at least 15 minutes before adding the toppings. Add the nutella to a microwave-safe container, and microwave for a few seconds so that it's easier to work with. Use a rubber spatula to smooth nutella on the top of each cookie.
⅔ cup nutella, ¾ cup chopped hazelnuts**
Sprinkle each with chopped hazelnuts.Enjoy!