Melt the butter in a pot over medium heat. Once completely melted, the butter will crack/sizzle for a few minutes. Let it do this while stirring occasionally.
211 grams unsalted butter (European style is preferred)*
Once the butter stops rapidly bubbling, it will begin to foam up in the pan. Gently stir until you can see golden milk solids floating in the butter & the butter is golden brown.
Remove from the heat & pour the butter into a different container. Since the butter is so hot in the pan, it will continue cooking. Transferring the butter to another container will stop it from browning further and potentially burning.
While the butter cools, coarsely chop the chocolate & pistachios and set aside.
150 grams bittersweet chocolate, 90 grams roasted and salted pistachios
Once the butter is cool enough (it can be warm, not hot), whisk together the butter, dark brown sugar, granulated sugar, and vanilla extract.
150 grams dark brown sugar*, 50 grams granulated sugar, ½ tablespoon vanilla extract
Add the egg & sour cream and whisk until combined. Whisk in the baking soda, baking powder, cornstarch, and salt.
1 egg, 20 grams sour cream, 1 teaspoon baking soda, 1 teaspoon baking powder, 1 teaspoon cornstarch, ½ teaspoon fine sea salt
Add the flour and fold until a few streaks of flour are remaining. Add the chocolate & pistachios and fold until fully incorporated & the flour is fully mixed in.
225 grams all purpose flour
Cover the bowl & chill the dough for at least three hours. Once ready to bake, preheat the oven to 350F and line a baking sheet with parchment paper.
Scoop the dough using a medium cookie scoop (about 3 tablespoons or 45-50 grams) and space them 2-3 inches apart on your baking sheet.
Bake for 10-11 minutes until the edges are set and the centers still look slightly underdone.
Let cool on the baking sheet for 5 minutes, then transfer to a cooling rack until cool enough to handle or let cool completely.