Line three rimmed baking sheets with silicone macaron baking mats to prevent sticking.
In the work bowl of a food processor, pulse superfine blanched almond flour, confectioners’ sugar, and Dutch process cocoa powder until finely ground. Sift the mixture through a large fine-mesh sieve into a large bowl, discarding any solids. This step is crucial for a smooth batter.
Add two of the egg whites (60 grams) to the dry mixture and fold until combined. Cover this mixture with plastic wrap to prevent it from drying out.
In a small saucepan, combine granulated sugar and water. Cook over high heat without stirring until an instant-read thermometer reads between 240°F (116°C) and 245°F (118°C). This creates a sugar syrup essential for the meringue.
In the bowl of a stand mixer fitted with the whisk attachment, beat the remaining two egg whites (60 grams) with cream of tartar at low speed until foamy.
Increase the mixer speed to medium-high and whip until medium-stiff peaks form. You want the egg whites to be glossy and hold their shape well.
With the mixer on high speed, gradually add the hot sugar syrup to the whipped egg whites, pouring it in carefully to avoid the sides and whisk. Continue to beat until the mixture is cool and forms stiff peaks, about four minutes.
Now, add one-fourth of the meringue (about ½ cup or 60 grams) to the flour mixture and stir vigorously until combined. This helps to loosen the batter.
Carefully fold in the remaining meringue until the mixture reaches a ribbon-like consistency. The batter should be thick yet flow smoothly off the spatula.
Transfer the batter to a pastry bag fitted with a medium round piping tip (Ateco #804). Pipe the batter into circles on the prepared baking mats. Hold the bag with the tip perpendicular to the mat, applying even pressure as you fill each circle.
Once piped, vigorously slam the pans on the counter five to seven times to release air bubbles trapped in the batter.
Sprinkle the tops of each macaron with finely chopped hazelnuts. Let the macarons stand at room temperature for 30 to 45 minutes, or until they form a skin that is dry to the touch.
Preheat your oven to 300°F (150°C).
Bake the macarons until they are firm but still have a slight wobble, about 13 to 15 minutes. They should not brown.
Allow the shells to cool completely on wire racks before removing them from the baking mats.
Prepare your Hazelnut-Chocolate Ganache and transfer it to a pastry bag fitted with a medium round piping tip. Pipe the ganache onto the flat side of half the macaron shells.
Top with the remaining shells, flat side down, to create your sandwiches. Store them in an airtight container in the refrigerator for up to three days.