Preheat oven to 375˚F. Line two large baking sheets with silicone mats or parchment paper.
In a medium bowl, whisk together flour, baking powder, salt, and baking soda until combined. Set aside.
Add butter, brown sugar, and sugar to the bowl of a stand mixer fitted with a paddle attachment. Beat on medium speed until light and fluffy, about 3 to 4 minutes. Beat in egg yolks, egg, and vanilla. Beat until mixture is pale and fluffy, about 4 to 5 minutes. Scrape down the sides as needed.
Reduce mixer speed to low. Gradually beat in dry ingredients, until just blended. Fold in chocolate and pistachios with a spatula.
Spoon rounded tablespoons of dough onto baking sheets. Space the dough balls about 1 inch apart.
Sprinkle each cookie with flaky salt.
Bake cookies until golden brown on the edges, rotating halfway through, for 10 to 12 minutes. Let cool for 5 minutes on baking sheets. Move to wire racks to cool completely.