Preheat your oven to 300°F and line two baking sheets with parchment paper.

In a bowl, sift together the powdered sugar, almond flour, and finely ground pistachios. Set aside.

In a separate bowl, beat the egg whites with an electric mixer on medium speed until frothy. Add the cream of tartar and continue beating until soft peaks form.

Gradually add the granulated sugar while beating on high speed until stiff peaks form and the mixture is glossy. Mix in the vanilla extract and a few drops of green food coloring if desired.

Gently fold the dry ingredients into the egg white mixture using a spatula. Be careful not to deflate the egg whites; fold until just combined and the batter flows like lava.

Transfer the batter to a piping bag fitted with a round tip. Pipe 1.5-inch circles onto the prepared baking sheets, leaving space between each macaron. Tap the baking sheets gently on the counter to release any air bubbles.

Let the piped macarons sit at room temperature for 30-60 minutes until a skin forms on the surface.

Bake the macarons for 15-18 minutes, rotating the sheets halfway through. They should have a smooth top and a slight foot.

Allow the macarons to cool completely on the baking sheets before removing them.

For the filling, beat the softened butter until creamy. Gradually add the powdered sugar and vanilla extract, mixing until smooth. Stir in the chopped pistachios.

Pair the macaron shells and pipe a small amount of filling onto the flat side of one shell, then sandwich it with another shell. Repeat with the remaining shells.

For best flavor, refrigerate the filled macarons for at least 24 hours before serving. Bring to room temperature before enjoying.