In a medium saucepan, melt 1 cup of unsalted butter over medium heat. Once melted, continue to cook, stirring frequently. The butter will begin to foam and then turn a golden brown color. You’ll know it’s ready when you see brown specks forming at the bottom of the pan and it emits a nutty aroma. Immediately remove the pan from the heat to prevent burning and transfer the brown butter to a large mixing bowl. Let the brown butter cool for 10 minutes.

Add 1 ¼ cups of brown sugar and 1 ¼ cups of granulated sugar to the cooled brown butter. Mix thoroughly for about 2 minutes until fully combined. Add in 2 eggs, 1 egg yolk, and 1 tablespoon of vanilla extract. Mix again for about 1-2 minutes until smooth and creamy.

In a separate bowl, whisk together 3 cups of all-purpose flour, 1 teaspoon of cornstarch, 1 teaspoon of baking soda, 1 teaspoon of baking powder, and 1 teaspoon of salt.

Gently fold the dry ingredients into the wet mixture until just combined. Add 1 cup of chocolate chunks and 1 cup of semi-sweet chocolate chips to the dough. Mix until evenly distributed.

Preheat your oven to 350°F (175°C). Transfer the cookie dough into a Quarter Sheet Pan (9"x 13") lined with parchment paper or lightly greased. Spread the dough evenly across the sheet. Bake for 18 to 22 minutes, or until the edges are slightly golden brown. Avoid over-baking to keep the brown butter chocolate chunk sheet pan cookies soft and chewy.

Allow the cookies to cool completely in the baking sheet. Optional, but highly advised, top with flakey sea salt. Once cooled, slice the brown butter chocolate chunk sheet pan cookies into desired pieces and serve.