To make the macaron cookie, cover several baking sheets with parchment paper. Prepare your pastry bag by placing a round pastry tip in the bottom and place in a large glass or vase to make it easier to fill.
Add half of the powdered sugar to a food processor fitted with the blade attachment and top with the almond flour and then the remainder of the powdered sugar and the cocoa powder. Pulse about 6 to 8 times until no almond flour is visible. Sift several times, discarding any large bits left in the sifter. Set aside.
In an electric stand mixer fitted with the whisk attachment, add the egg whites and cream of tartar. Make sure the bowl and whisk are freshly cleaned and dried. Whisk on medium until the eggs are frothy and then slowly add in the granulated sugar. Turn the mixer on high until you reach stiff peaks. You’ll know it’s ready when you invert the whisk and the meringue holds its shape. The tip of the inverted meringue will be just shy of 12 o’clock. It should also feel smooth when you rub some between your fingers.
Sift the dry ingredients one more time into the meringue in three batches, folding and mixing between each one. It should take around 20 to 25 folds for everything to get mixed together. Then spread the batter up the sides of the bowl and scoop it back down to the bottom. Repeat this about three or four times or until the mixture starts to slowly slump back down the sides of the bowl. Scoop the mixture into your prepared pastry bag and cut the bottom tip off.
Hold the pastry tip just above the baking sheet, and then press out the batter into the shape of a circle the size of a nickel, pause for a second at the top and then quickly pull the pastry tip off to the side. You want the tops as smooth as possible. The macarons will spread a wee bit so leave about 1 1⁄2 to 2 inches (3.7 to 5 cm) between each. Tap the pans 4 times on the counter to release any trapped bubbles. If there are any bubbles near the surface that don't pop, use a toothpick to pop them. Let the macarons sit for 20 to 45 minutes until they become dull, and when gently touched no batter is transferred to your finger; this time will vary depending on local humidity.
Preheat the oven to 320°F (160°C) and bake the macarons for 11 minutes, or until they lift easily off of the parchment. Take care not to overbake them, they shouldn’t darken or show any color change in the oven. Let cool completely.