Before you begin get all the ingredients ready. Also prepare your baking sheets by lining them with parchment paper.
Start by combining confectioners sugar, almond flour, and cocoa in a food processor. Pulse a few times to get it mixed good. Set aside.
Whisk egg whites until they start to get foamy. Add cream of tartar and whisk until soft peaks form. Reduce speed to low and add granulated sugar, once its mixed in, increase speed to high and beat until stiff peaks form (about 3 minutes).
Sift the flour mixture into the egg whites (do not use a flour sifter, a regular sieve will work better) fold the mixture until it is smooth.
Transfer the mixture into a piping bag fitted with a large round tip. ( Wilton 1a or 2a depending on the size you want) Pipe macaron mixture onto prepared baking sheet about an inch in between each one. Let sit about 30 minutes or until you can gently touch the top and it won't stick to you finger. Meanwhile preheat oven to 325 degrees. When they don't stick to your finger pop them in the oven for about 10 minutes. Take them out and test for doneness by gently trying to slide the top of the macaron. It should just barely move. Let cool while you prepare the filling.
For the filling pour the heavy cream in a small saucepan over medium heat. Heat until it simmers. Place the chocolate and nutella in a heat safe bowl. Pour the heavy cream in the bowl and wait a minute or so. Then whisk the mixture until everything is melted. Place in the refrigerator for 20-30 minutes for it to stiffen up.
Once the macarons are ready to be assembled, take out the ganache and beat it on high until it changes to a lighter color. Put the macarons together by scooping the ganache on the tip of a teaspoon and placing in the center of one macaron. Take the second macaron and gently press the two together until the ganache comes out to the edge.
Store in the refrigerator. Let sit on counter for about 10-15 minutes before serving. Enjoy!