Sift together 1 cup of almond flour, ½ cup of pistachio flour, and 1¾ cups of powdered sugar in a large bowl. Set aside.

In a clean mixing bowl, add 3 large egg whites and ¼ teaspoon of cream of tartar. Beat until foamy, then gradually add ¾ cup of granulated sugar. Whip until stiff peaks form, about 6-8 minutes.

Gently fold the sifted dry ingredients into the whipped egg whites. Add 1 teaspoon of almond extract and a few drops of green food coloring.

Transfer the batter into a piping bag fitted with a round tip. Pipe small circles onto a parchment-lined baking sheet, spaced about an inch apart.

Let the piped macarons rest at room temperature for about 30 minutes. Preheat your oven to 300°F (150°C).

Bake the rested macaron shells in the preheated oven for 15-20 minutes. They should not brown.

While the shells cool, prepare the filling by beating ½ cup of softened unsalted butter until fluffy. Gradually add 1½ cups of powdered sugar and ½ cup of pistachio paste.

To assemble, match shells of similar sizes and pipe a generous dollop of filling onto one shell, then press another shell on top.