1. Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper.
2. In a medium bowl, whisk together the flour, baking soda, and salt. Set aside.
3. In a large bowl, beat the softened butter, brown sugar, and granulated sugar until creamy and well combined, about 2 minutes.
4. Beat in the egg, egg yolk, and vanilla extract until smooth. Scrape down the sides of the bowl as needed.
5. Add the dry ingredients to the butter mixture. Mix just until a soft dough forms. Do not overmix.
6. Fold in the white chocolate and chopped pistachios, reserving a few pieces for topping if desired.
7. Cover the dough and refrigerate for 30 minutes.
8. Scoop the dough into balls, about 2 tablespoons each, and place them on the prepared baking sheets, leaving about 2 inches between cookies.
9. Press a few extra pieces of white chocolate and pistachios onto the tops if desired.
10. Bake for 10–13 minutes, until the edges are lightly golden and the centers still look slightly soft.
11. Let the cookies cool on the baking sheet for 5 minutes, then transfer them to a wire rack to cool further.