Line two baking sheets with silicone mats or parchment paper; set aside. Fit a large pastry bag with a 1/2-inch round tip; set aside.
CHOCOLATE MACARONS: 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 on 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 over whip.) 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 just combined. 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 over mix.)
Transfer the batter to the pastry bag. Pipe 1-1/4-inch rounds, about 1 inch apart onto the prepared 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 foot. Let the rounds sit at room temperature for 30 minutes to dry the tops and ensure even cooking.
Heat the oven to 350 degrees and arrange a rack in the middle. Bake the Macarons one 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.
WHITE CHOCOLATE PEPPERMINT CREAM: In a small microwavable bowl, combine chopped chocolate, cream and butter; microwave in 30 second intervals, stirring, until smooth. Stir in peppermint extract. Allow to cool a bit.
To assemble the Macarons, take two cookies and sandwich them together with cream filling. While you can serve the Macarons the day they are made, these cookies really need to mature for a day or two in the refrigerator. Bring to room temperature before serving.