Place an oven rack on the center shelf and preheat the oven to 375 degrees F. Line 2-4 baking sheets with parchment paper. (You’ll be baking in batches).

Place the butter, brown sugar and granulated sugar in a stand mixer fitted with the paddle attachment (or you can use a large mixing bowl with an electric hand mixer). Beat the ingredients on medium-high speed until pale and fluffy (3 to 5 minutes).

Add the egg, egg yolk, and vanilla, beating until completely incorporated. Scrape down the bottom and sides of the bowl with a silicone spatula

In a large bowl, whisk together the flour, cornstarch, baking soda, and salt. Add to the mixer in batches, pulsing on low first, then mixing on medium-low speed until all the ingredients are combined (do not over-mix).

Stir in the chocolate chips and pistachios. Cover the bowl, place in the refrigerator, and chill for 45 minutes.

Use a 2-tablespoon scoop to drop balls of cookie dough onto the prepared sheets, leaving about 1 1/2 - 2 inches in between. Sprinkle with the flaked sea salt.

Bake one sheet at a time until set around the edges are just barely beginning to turn golden, approximately 8 to 10 minutes.

Cool completely on the baking sheet, then transfer to an air-tight container for storage.