Cube the butter and place in a light-colored skillet. Melt the butter over medium heat, stirring constantly.

Once melted, the butter will begin to foam. Keep stirring. After 5-8 minutes, the butter will begin browning; you’ll notice lightly browned specks begin to form at the bottom of the pan and you'll begin to smell a nutty aroma. Once browned, immediately remove from heat, pour into a heat-proof bowl, and set aside for 10 minutes to cool slightly. You should have 7 tablespoons (98 grams) of brown butter.

To the brown butter (which should still be liquified), add the brown sugar and granulated sugar and whisk until combined. Add the egg and vanilla and whisk again until well combined.

Add the flour, baking soda, and salt to the wet ingredients and mix until just combined; be sure not to overmix! Fold in ½ cup (75 grams) of pistachios and most of the chocolate chunks (reserve some for the next step) until incorporated.

Using a 1½-tablespoon spring-loaded scoop, scoop the dough into balls and place dough balls on a lined baking sheet. Top with remaining chocolate chunks then loosely cover with plastic wrap, and place in the refrigerator to chill for 30-60 minutes.

Preheat the oven to 350°F (180°C). Line another baking sheet with parchment paper and set aside.

Bake the cookies for 10-12 minutes, or until lightly browned on the sides; the centers will look very soft. Remove from the oven and sprinkle the tops with remaining chopped pistachios and flaky salt. Allow to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.