In a medium saucepan over the lowest heat possible, melt 1 cup unsalted butter. There should be no sizzle, crackling or popping — just let the butter ooze into liquid without boiling, ensuring minimal moisture is lost. Stir regularly, until the butter is almost completely melted.
In a bowl, whisk together 3 1/4 cups all-purpose flour, 1 1/4 teaspoons baking powder, 1 teaspoon baking soda, and 1 1/2 teaspoons kosher salt. Set aside.
Once the butter is melted, pour it into a large bowl and whisk in 1 1/2 cups dark brown sugar and 1/2 cup granulated sugar. The mixture may look like it will seize, but it will relax with a few seconds of stirring. Add 2 large eggs, one egg at a time, whisking briefly after each addition but only to combine. Stir in 2 teaspoons pure vanilla extract.
Once the vanilla extract is incorporated into the mixture, switch from a whisk to a rubber spatula. Sprinkle the dry ingredients (from the 2nd step) over the mixture, and use the rubber spatula to mix in the dry ingredients. Once mostly blended (think: two or three smallish flour streaks left in the mixture), stir in 12 ounces coarsely chopped bittersweet feves. Be sure to bring any stray ingredients up from the bottom of the bowl, and continue mixing until the dough no longer looks dusty. If the dough seems warm or overly glossy, refrigerate for 5 minutes. Use a 1-tablespoon cookie scoop dough to measure out 3 tablespoons of dough for each cookie, using your hands to roll together to create a bigger ball of dough to make one cookie.