In a large bowl, whisk together flour, granulated sugar, yeast, and salt. Add the warm milk and stir until a shaggy dough forms. Knead on a floured surface for 5-7 minutes until smooth and elastic. Wrap in plastic and refrigerate for 30 minutes.

Roll the chilled dough into a 10x16-inch rectangle on a floured surface. Arrange the cold butter slices in an even layer over two-thirds of the dough. Fold the unbuttered third over the middle, then fold the opposite third over that, like a business letter. Rotate 90 degrees, roll into a 10x16-inch rectangle again, and repeat the letter fold. Wrap and refrigerate for 1 hour.

Repeat the rolling and folding process two more times, chilling for 30 minutes between each turn. After the final fold, refrigerate the dough for at least 2 hours or overnight for easier handling.

For the pistachio cream, blend the toasted pistachios in a food processor until finely ground. Add heavy cream, powdered sugar, and vanilla extract, then blend until smooth and spreadable, about 1-2 minutes. Transfer to a bowl and refrigerate until ready to use.

Roll the laminated dough into a 12x20-inch rectangle about 1/4-inch thick. Cut into 8 rectangles (approximately 5x6 inches each). Place 2 tablespoons of pistachio cream in the center of each rectangle, spreading slightly. Fold the dough over the filling to create a pocket, pressing edges firmly to seal.

Place the filled croissants seam-side down on parchment-lined baking sheets, spacing them 3 inches apart. Brush lightly with egg wash, cover loosely with plastic wrap, and let proof at room temperature for 2-3 hours until nearly doubled in size and visibly puffy.

Preheat oven to 400°F. Brush the proofed croissants with a second coat of egg wash. Bake for 18-22 minutes, rotating the pans halfway through, until deep golden brown and flaky. Cool on the pans for 5 minutes, then transfer to a wire rack. Serve warm or at room temperature.