Brown the Butter: Melt unsalted butter in a medium saucepan over medium heat, stirring frequently for about 5 minutes until golden brown and nutty aroma is released. Let cool for 30 minutes.

Mix Wet Ingredients: In a large mixing bowl, combine the cooled brown butter with light brown sugar and white sugar, whisking until smooth and creamy. Add the egg and vanilla extract, mixing until well combined.

Combine Dry Ingredients: In another bowl, whisk together flour, baking soda, and salt. Gradually fold this dry mixture into the wet ingredients until just combined, avoiding overmixing.

Add Mix-Ins: Gently fold in chopped pistachios and dark chocolate chunks into the dough, ensuring even distribution.

Scoop and Bake: Preheat oven to 350°F (175°C). Line a baking sheet with parchment. Scoop dough onto the sheet, leaving space in between. Bake for 12-15 minutes until edges are golden brown and centers are slightly soft.

Finishing Touches: Immediately sprinkle a pinch of flaky sea salt on each cookie after baking. Let cool briefly on the baking sheet, then transfer to a wire rack.