In a large bowl, whisk together the melted butter and granulated sugar until well combined.
Next, add the egg and vanilla and whisk to combine.
Add the flour and baking soda and stir just until no large lumps remain - make sure not to overmix.
Add the white chocolate chips and pistachios and fold them in gently.
Transfer the batter to the fridge to chill for 30 minutes. While the dough chills, preheat your oven to 375°F .
Line a large baking sheet with parchment paper or a silpat mat. Scoop one tablespoon sized balls onto the baking sheet, making sure to place them at least two inches apart.
Bake for 15 minutes, or until the edges of the cookies are set and just starting to brown - the center will still be soft.
Allow the cookies to cool fully on the tray. They will set as they cool.
Enjoy immediately or store any leftovers in an airtight container at room temperature for up to three days.