Make the ganache: Bring the heavy cream to a boil and pour it over the chopped white chocolate. Whisk the mixture until smooth, then add the butter. Chill in the refrigerator for 1 hour.

Prepare the macarons: In a food processor, combine the powdered sugar and almond flour, then sift the mixture. Whip the egg whites with a pinch of salt until soft peaks form, then add the pistachio paste for color and flavor. Gradually add the granulated sugar and continue whipping until stiff peaks form. Gently fold the sifted powdered sugar and almond flour mixture into the meringue with a spatula until the batter is smooth and flows in ribbons.

Preheat your oven to 320°F (160°C). Pipe small rounds of batter onto a baking sheet lined with parchment paper. Tap the baking sheet to smooth the tops. Let the macarons rest for 30 minutes to form a slight crust. Bake for about 12 minutes, then let cool. Fill a piping bag with the ganache and assemble the macarons.