Place a heavy-bottomed, light-colored pan over medium heat and add the butter. Stir continuously and keep the butter moving the entire process. When the butter starts melting, bright milk solids will start forming on the surface. Once the butter is completely melted, the majority of the milk solids sink down to the bottom, and the butter starts bubbling. After a few minutes, the butter will start foaming, and the milk solids browning. It will take a total of 5-7 minutes of cooking time once the butter is melted. You know when it's done when you notice a buttery and nutty aroma, and the color of the butter is golden brown with toasty brown milk solids.
Immediately remove the butter from the pan and transfer, including the toasted milk solids, to a heat-proof cup that is wide enough that you can easily pour in the butter without wasting some of it. Then pour the butter with the milk solids into a 1 cup (240ml) measuring cup and fill up with the milk until full. Transfer the liquid including the milk solids to a heat-proof container and chill in the fridge for 1 hour or up to 2 days. Before using the browned butter, let it sit at room temperature for 30 minutes to soften.
In a medium bowl, combine the flour, cornstarch, baking soda, and salt. Set aside.
In a large mixing bowl, using an electric mixer fitted with a paddle or whisk attachment, beat the butter on medium-high speed until soft and creamy, about 1-2 minutes. Add the brown sugar and sugar and cream for 2 minutes. Then add the eggs and vanilla and mix until well combined and creamy, about 1 minute. Add the flour mixture and mix on medium-low speed just until incorporated, about 30-60 seconds. Then stir in the toasted pecans and chocolate chips to incorporate, if used. Cover and chill for 2 hours or up to 2 days. After chilling, allow the dough to come to room temperature for 15-30 minutes until it is a soft and spoonable dough.
Line 2-3 baking sheets with parchment paper and preheat the oven to 350°F / 175°C.
Scoop 24 equal-sized balls of cookie dough (2 tablespoon / 45g each) and roll into even balls. Place 6-8 cookies, spaced about 2 inches (5cm) apart, on each baking sheet and bake one sheet at a time for 10-11 minutes or until the edges are lightly browned and the center looks puffy, pale, dry, and underbaked.
Remove from the oven and let cool for 5 minutes or until firm enough to transfer them to a wire rack. They will continue baking on the baking sheet. Then let cool completely. Store in an airtight container at room temperature for up to 4 days or freeze for up to 3 months.