In a large mixing bowl, cream together the softened unsalted butter, granulated sugar, and light brown sugar using an electric mixer on medium speed until the mixture is light and fluffy, about 3-4 minutes. Scrape down the sides of the bowl as needed. This process incorporates air, which contributes to the cookie's texture.

Add the two large eggs one at a time, beating well after each addition until fully incorporated. Then, mix in the vanilla extract. Ensure each egg is thoroughly combined before adding the next to prevent the dough from becoming oily.

In a separate medium bowl, whisk together the all-purpose flour, cornstarch, baking soda, and salt. Whisking ensures the leavening agents and salt are evenly distributed throughout the dry ingredients, preventing pockets of saltiness or uneven rising in the cookies.

Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix, as this can develop the gluten in the flour and result in tough cookies. Stop mixing as soon as you no longer see streaks of flour.

Gently fold in the roughly chopped roasted pistachios and the semi-sweet chocolate chunks using a spatula or wooden spoon. Distribute them evenly throughout the dough. Reserve a small handful of pistachios and chocolate chips to press onto the tops of the cookies before baking for a more appealing look.

Cover the bowl with plastic wrap and refrigerate the dough for at least 1 hour, or up to 24 hours. Chilling the dough is crucial for developing flavor and ensuring the cookies don't spread too much during baking, resulting in a thicker, chewier cookie.

Preheat your oven to 190°C (375°F). Line two large baking sheets with parchment paper or silicone baking mats. This prevents the cookies from sticking and ensures even baking.

Scoop rounded tablespoons of chilled dough (about 2 tablespoons or 30g each) and place them about 5cm (2 inches) apart on the prepared baking sheets. If you chilled the dough for a long time, it might be quite firm. You can gently roll them into balls, but avoid overworking the dough.

Gently press a few reserved pistachio pieces and chocolate chunks onto the tops of each dough ball to mimic the look of the interior chunks. This step is purely for aesthetics but makes the cookies look more professional.

Bake for 10-12 minutes, or until the edges are lightly golden brown and the centers are still slightly soft and appear slightly underbaked. The cookies will continue to set as they cool. Do not overbake if you desire a soft and chewy texture.

Let the cookies cool on the baking sheets for 5-10 minutes before transferring them to a wire rack to cool completely. This allows them to firm up enough to be moved without falling apart.

Store cooled cookies in an airtight container at room temperature for up to 3 days for optimal freshness and chewiness.