Line baking sheets with parchment paper or silicone mats (a macaron template is optional). Preheat nothing yet.

In a food processor combine 130 grams almond flour, 120 grams powdered sugar, and 10 grams unsweetened cocoa powder. Pulse (blitz) about 16 times to combine.

Sift the processed dry mixture into a bowl. Discard any coarse bits left in the sifter. Set the sifted dry mixture aside.

Ensure your glass or metal mixing bowl and beaters are completely dry and grease-free. Put 100 grams room-temperature egg whites (about 3 large egg whites) and 1/4 teaspoon cream of tartar into the bowl.

Using a hand mixer on high speed, beat the egg whites and cream of tartar until frothy, about 1 minute.

With the mixer running on high, slowly add 90 grams granulated sugar. Continue beating on high until you reach stiff peaks (when you lift the beater the peaks stand straight up and do not fold over). Do not overbeat.

Fold one-half of the sifted dry mixture into the meringue: use a flexible spatula to cut around the outside of the bowl and fold through the center. Continue folding until no dry streaks remain.

Add the remaining sifted dry mixture and continue folding. Work until the batter reaches a molten-lava consistency: it should be smooth and flow slowly off the spatula, and you should be able to draw a figure-8 that does not break apart. Stop folding as soon as you reach this stage to avoid overmixing.

Transfer the batter to a pastry bag fitted with a large round tip (for example Wilton 2A). Tap the bag to remove air.

Pipe 1 1/2–2 inch (3.8–5 cm) even circles onto the prepared baking sheets, spacing them about 1 inch (2.5 cm) apart.

Gently tap the bottom of each baking sheet on the counter 5–6 times to release large air bubbles. Pop any remaining large bubbles on the surface with a toothpick (move the toothpick in a circular motion to drag the bubble out).

Let the piped macarons rest at room temperature for 45 minutes to 1 hour until they develop a skin: the tops should no longer be sticky and should feel firm to the touch (no batter should come off on your finger).

Preheat the oven to 320°F (160°C) while the shells finish drying.

Bake the macarons, one sheet at a time if necessary, for 15–17 minutes. The macarons are done when the tops are set and you can gently touch the top without the shell moving on the feet.

Remove the baking sheet from the oven and allow the macarons to cool completely on the sheet before trying to remove them.

While the shells are cooling, make the filling: in the bowl of a stand mixer fitted with the paddle (or using a hand mixer), beat 1/2 cup softened unsalted butter and 1/2 cup Nutella on high speed for 3–4 minutes until smooth.

With the mixer on low, gradually add 1 cup powdered sugar until just combined, then add 2 tablespoons heavy whipping cream, 1 teaspoon pure vanilla extract, and a pinch of salt.

Increase the mixer speed to high and beat the filling for 3–4 minutes until light and whipped in texture. Transfer the buttercream to a pastry bag fitted with a large round tip.

Melt 1/2 cup semisweet chocolate chips until smooth (microwave in short bursts, stirring between intervals, or use a double boiler). Transfer the melted chocolate to a small resealable bag or piping bag and snip a small corner for drizzling.

To decorate half of the macaron shells: flip those shells over so the flat side is up, drizzle melted chocolate over each, and, if desired, sprinkle with crushed hazelnuts. Let the drizzle set.

Assemble the macarons: flip the remaining (non-drizzled) shells over so the flat sides face up. Pipe a generous circle of Nutella buttercream onto each non-drizzled shell, then top with a drizzled shell to form sandwiches.

Chill assembled macarons for at least 24 hours in an airtight container if possible for best texture, or serve after the filling has set. Store finished macarons in an airtight container in the refrigerator.