In a medium saucepan over medium heat, melt the unsalted butter. Continue to cook, stirring frequently, until the butter turns golden brown and has a nutty aroma. This usually takes about 5-7 minutes. Be careful not to burn it. Once browned, remove from heat and let it cool slightly.
In a large mixing bowl, whisk together the browned butter, light brown sugar, granulated sugar, and vanilla extract until well combined. Add the large egg and mix until smooth and creamy.
In a separate bowl, whisk together the all-purpose flour, cornstarch, baking soda, and salt. This helps to ensure that the leavening agents are evenly distributed throughout the flour.
Gradually add the dry ingredients to the wet mixture, stirring just until combined. Be careful not to overmix, as this can lead to tough cookies.
Gently fold in the semi-sweet chocolate chips and peanut butter chips using a spatula. Ensure they are evenly distributed throughout the dough.
Cover the bowl with plastic wrap and refrigerate the dough for at least 30 minutes. Chilling helps the cookies maintain their shape and enhances the flavors.
Preheat your oven to 350°F (175°C) while the dough chills.
Using a cookie scoop or tablespoon, drop rounded balls of dough onto a baking sheet lined with parchment paper, leaving space between each cookie as they will spread.
Bake in the preheated oven for 10-12 minutes, or until the edges are golden and the centers are set. They may look slightly underbaked, but they will continue to cook on the baking sheet once removed from the oven.
Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a cooling rack to cool completely.
