In a mixing bowl, combine the shelled pistachios, powdered sugar, heavy cream, vanilla extract, and salt. Whisk until smooth and creamy.

Preheat the oven to 400°F (200°C). Roll out the puff pastry on a lightly floured surface and cut it into triangles.

Place a spoonful of pistachio cream at the wide end of each triangle and roll it up tightly towards the point.

Place the rolled croissants on a baking sheet lined with parchment paper.

In a small bowl, mix the beaten egg with milk. Brush the mixture over the croissants for a golden finish.

Bake in the preheated oven for 15-20 minutes or until golden brown and puffed.

Let the croissants cool slightly before serving. Enjoy warm or at room temperature.