Line 2 large baking sheets with parchment paper or silicone mats; set aside. Combine the dry ingredients - almond flour, confectioner's sugar and cocoa powder - and sift into a large mixing bowl. In a separate bowl, whisk together the egg whites and granulated sugar for 3 to 4 minutes until stiff peaks form.
Transfer the meringue into the dry ingredients and then begin folding. Fold and flatten the meringue, combining it with the dry ingredients, until a thick batter forms. The batter should just ribbon off the spatula. Transfer the cookie batter into a pastry bag tipped with a simple, round tip. Pipe even sized dollops onto the prepared paper or mats, spacing the cookies about an inch apart.
Once the pan is filled, sprinkle each cookie with chopped hazelnuts. Tap the pan a few times against the work surface to release any trapped air bubbles. Allow the cookies to rest at room temperature for 1 hour, allowing a thin shell to form on the outside. Meanwhile, preheat the oven to 320F. Bake the cookies for 18 to 19 minutes; cookies should come off the paper with ease. Remove them from the oven and cool completely on the pan.
While the shells are cooling, prepare the filling. In a large mixing bowl, whisk the softened butter for a few minutes until it's fluffy. Add the vanilla, salt, cocoa powder and Nutella, then mix again. Add the confectioner's sugar last. Once it's incorporated, whisk the filling on high speed for a few minutes until the filling is fluffy. Transfer the filling into a pastry bag tipped with a star tip.
Once macaron shells are cooled, melt 1 cup of chocolate melts in the microwave or over a double boiler. Pair the cookies up according to size, then dip the cookies into the chocolate half way, tapping off any excess. Set the cookies back down onto the paper or silicone and allow the chocolate to set. Place the cookies into the fridge for faster setting.
To assemble the cookies, simply pipe a generous amount of the filling onto the center of one shell, then top with a second one. Place the cookies into an airtight container and refrigerate the cookies for at least 1 to 2 days. When ready to enjoy, bring the cookies up to room temperature first.