1. In a large bowl, combine the flour, chilled butter, granulated sugar, and salt. Use a pastry cutter or two forks to cut the butter into the dry ingredients until the mixture resembles coarse crumbs.

2. Add the chilled milk and stir just until a shaggy dough forms. Turn the dough out onto a lightly floured surface and knead briefly, about 1 minute, until it comes together.

3. Shape the dough into a disk, wrap in plastic wrap, and refrigerate for at least 1 hour (or up to 2 days).

4. In a food processor, blend the chopped pistachios and heavy cream until a smooth, creamy filling forms. Stir in the powdered sugar and set aside.

5. On a lightly floured surface, roll the chilled dough out into a large rectangle, about 1/4 inch thick. Fold the dough in half, then in half again to create 4 layers.

6. Cut the folded dough into 8 equal triangles. Place a heaping tablespoon of the pistachio cream filling onto the wide end of each triangle.

7. Starting from the wide end, tightly roll up each triangle into a croissant shape. Place the filled croissants onto a parchment-lined baking sheet, seam side down.

8. Brush the tops of the croissants with the beaten egg.

9. Bake at 375°F for 18-22 minutes, until the croissants are golden brown.

10. Remove from the oven and let cool for 5 minutes before serving.