01 - Using a food processor, grind pistachios until finely ground. Add butter and powdered sugar; blend until mixture is smooth. Incorporate egg, vanilla extract, flour, and salt, blending until creamy. Refrigerate the mixture for at least 1 hour for optimal consistency.
02 - Set the oven to 350°F and line a baking sheet with parchment paper.
03 - In a small bowl, combine whole milk and granulated sugar, stirring until dissolved.
04 - With a sharp knife, cut each croissant horizontally, leaving one edge attached as a hinge.
05 - Use a pastry brush to lightly coat the interior of each croissant with the simple syrup.
06 - Distribute 2 to 3 tablespoons of pistachio cream inside each croissant, then gently close them.
07 - Spread a thin layer of pistachio cream on top of each croissant and sprinkle with chopped pistachios.
08 - Place croissants on the prepared sheet and bake for 12 to 15 minutes, until croissants are golden and the cream is set.
09 - Allow croissants to cool slightly. Dust with powdered sugar before serving if desired.