Preheat oven to 350°F. Line baking sheets with parchment paper and set aside.
In a large bowl, add melted butter, both sugars, and vanilla and whisk to combine.
1 cup unsalted butter, 1 cup light brown sugar, ½ cup granulated sugar, 1 Tablespoon vanilla extract
Add eggs and whisk well until the mixture is glossy and a bit lighter in color, about 1 minute. Let the mixture stand for about 5 minutes while you prepare the next ingredients.
1 large egg plus 1 large egg yolk
In a medium bowl, add flour, cornstarch, baking soda, and salt and whisk to combine.
2 ¼ cups all-purpose flour, 2 teaspoons cornstarch, 1 teaspoon baking soda, 1 teaspoon fine sea salt
Give the sugar mixture a quick whisk and then add the flour mixture. Use a wooden spoon or a spatula to gently stir the flour mixture into the sugar mixture. Stir just until mixed.
Add the chopped chocolate and pistachios and stir just until blended.
6 ounces semisweet or bittersweet chocolate baking bar, 1 cup lightly salted shelled pistachios
Scoop 1 ½ Tablespoon balls of dough onto parchment-lined baking sheets. (Tip: For extra pretty cookies, add extra bits of chopped chocolate and nuts on top of the cookie dough balls.)
Bake the cookies for 10 to 12 minutes. The cookies will still appear pale and puffy when you remove the tray from the oven. Sprinkle a pinch of salt onto each cookie and allow them to cool on the tray for 5 minutes before transferring to a wire rack to cool completely.
Notes
Storage: These cookies taste best on the day they're baked, but they'll stay fresh for up to 3 days stored in an airtight container at room temperature.Dough can also be frozen and baked at a later date. Check out my tips for freezing cookie dough.