Preheat oven to 350°F. Line two baking sheets with parchment.
Whisk flour, cornstarch, baking soda, baking powder, and fine sea salt in a medium bowl.
Cream butter, brown sugar, and granulated sugar until pale and fluffy, about 2 to 3 minutes.
Beat in eggs one at a time, then mix in vanilla until smooth.
Add dry ingredients to the bowl and mix on low just until no dry streaks remain.
Fold in chopped pistachios and chocolate chunks until evenly distributed.
Scoop 2-tablespoon portions onto prepared sheets, spacing about 2 inches apart. Lightly sprinkle with flaky salt.
Bake 10 to 12 minutes, until edges set and centers look slightly underdone.
Cool 5 minutes on the sheet, then move cookies to a rack. Add a tiny pinch more flaky salt if desired.