For the Macarons: Line 2 baking sheets with parchment paper; set aside. Fit a large pastry bag with a 1/2-inch plain tip; set aside.

Place the powdered sugar, almond flour, cocoa powder, and salt in a food processor fitted with a blade attachment and pulse several times to aerate. Process until fine and combined, about 30 seconds. Sift through a flour sifter into a large bowl; set aside.

Make a meringue by placing the egg whites in the clean bowl of a stand mixer fitted with a whisk attachment. Beat in medium speed until opaque and foamy, about 30 seconds. Add the cream of tartar, increase the speed to medium high, and beat until the egg whites are white in color and hold the line of the whisk, about 1 minute. Continue to beat, slowly adding the granulated sugar, until the sugar is combined, the peaks are stiff, and the whites are shiny; about 1 minute more. (Do not overwhip.) Transfer the meringue to a large bowl.

Using a rubber spatula, gently fold the dry mixture into the egg whites in four batches until the dry ingredients are combined. (The meringue will deflate.) With the final addition, stop folding when there are no traces of egg whites, the mixture runs like slow-moving lava, and it looks like cake batter. (Do not overmix.)

Transfer the batter to the pastry bag. Pipe out 1-1/4-inch rounds about 1 inch apart ontp the baking sheets, about 25 per sheet. Pick up the baking sheets and bang them against the work surface to help create the macaron base, or the foot. Let the rounds sit at room temperature for 30 minutes to dry the tops and ensure even cooking.

Heat the oven to 350°F and arrange a rack in the middle. Bake the macarons 1 sheet at a time for 7 minutes. Rotate the sheet and cook for 7 minutes more. Transfer the sheet to a rack to cool completely.

For the Ganache Filling: place the chopped chocolate in a large bowl.

Warm the cream in a small saucepan ovee medium heat until it just starts to boil. Stir it into the chocolate without creating bubbles. Let sit for 1 minute. Add the butter and stir until smooth. Chill in the refrigerator until thickened but still spreadable, about 30 minutes.

To Assemble: Pair macarons of a similar size. Remove the ganache from the refrigerator. Squeeze or scoop the ganache to about the size of a cherry (about 1 teaspoon) onto the center of a macaron half. Top with another half and press gently so that it looks like a mini hamburger. The filling should not ooze out the edges. Refrigerate, covered, at least 24 hours before eating.