For the pistachio buttercream: In the bowl of a food processor, add the pistachios and powdered sugar. Process until the pistachios are very finely ground, scraping down the sides of the bowl once or twice during the process.
Place the butter in the bowl of a stand mixer fitted with the paddle attachment, or in a large mixing bowl if using a hand mixer, and beat for a minute to soften. Add the ground pistachio/sugar mixture and mix until fluffy and smooth. Add the pistachio extract and mix well. If the buttercream seems a bit stiff, add some heavy cream or milk by teaspoonfuls until you reach the right consistency. Transfer the pistachio buttercream to a pastry bag fitted with a small round tip and set aside until ready to assemble the macarons. (The buttercream can be made ahead; refrigerate, but make sure to return to room temperature at least an hour before using.)
For the pistachio macaron shells: **Return the egg whites to room temperature at least an hour before making the macaron shells.**
In the bowl of a food processor, add the powdered sugar, almond flour, and pistachios, and process until the mixture is thoroughly incorporated, 30 seconds to a minute. Sift the mixture in a fine mesh strainer to make sure no lumps or bigger bits of nuts are left. Discard the larger bits that remain in the strainer.
In the bowl of a stand mixer fitted with the whisk attachment, or in a large stainless steel mixing bowl if using a hand mixer, whisk the egg whites on medium/high speed until frothy, about 1 minute, then slowly pour in the granulated sugar. After 2 minutes, add the gel food coloring, if using. (For a light green, try a ratio of 6 drops yellow to 1 drop green, adding as much as needed to reach a shade you're looking for.)Keep beating until the egg whites are stiff, dense and creamy, about 3 minutes more.
Add the nut and powdered sugar mixture to the egg whites and, using a spatula, gently fold in the dry ingredients: slide your spatula all the way to the bottom of the bowl and comes back up to the top. Do this about 6 times to incorporate the dry ingredients, then keep folding for a total of about 14 times until no pockets of dry ingredients remain and the mixture drops from the side of the spatula in a slow, lazy ribbon. Start testing early to make sure not to overfold.
Stack two baking sheets and line the top sheet with parchment paper. If you're not using parchment paper sheets, cut out the parchment paper so it fits precisely over the bottom of the sheet to make sure the macarons will lay flat. Slide macaron templates under the parchment paper, if using.
Transfer the macaron batter to a pastry bag fitted with a ½-inch (1.25-cm) round tip. Pipe 1 ½-inch (3.8-cm) rounds of batter, evenly spaced but still close to one another as they will not expand much.
Carefully slide the macaron templates off the baking sheets, if you used them. Let the shells rest on the baking sheets for 20 to 30 minutes.
Preheat the oven to 275°F (135°C) with a rack set to the middle position. Bake each sheet of macaron shells for 13 to 16 minutes, or until the shells are firm on their feet when you lightly tap on them with the tip of a finger.
Let the shells cool completely to room temperature before assembling them, about an hour.
To assemble the Pistachio French Macarons: Pair same-sized shells together and set side by side on a work surface. Pipe some pistachio buttercream over half of the shells. Close the macarons, gently pressing the second shells over the filling.
STORAGE: Store the assembled Pistachio French Macarons in an airtight container and refrigerate for one night before indulging.French macarons will keep, refrigerated, for up to 3 days. You can also freeze assembled macarons in an airtight container for up to 1 month.
SERVING: Always bring these Pistachio French Macarons back to room temperature before serving.
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