French Chocolate Macarons with Nutella

Recipe adapted from www.chow.com
For the macarons

2 cups powdered sugar
1 cup almond flour or almond meal
3 tablespoons natural unsweetened cocoa powder
1/4 teaspoon fine salt
3 large egg whites, at room temperature
Pinch cream of tartar
3 tablespoons granulated sugar

For the Nutella filling

4 ounces bittersweet chocolate, finely chopped
1/2 cup heavy cream
2 tablespoons unsalted butter (1/4 stick), at room temperature and cut into cubes
2-3 tbs  Nutella

INSTRUCTIONS

For the macarons:

Line two 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 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 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 just 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 onto 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 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 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.

For the Nutella filling

Place the chopped chocolate in a large bowl.
Warm the cream in a small saucepan over 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.