Combine Wet Ingredients: In a large mixing bowl, whisk together the cooled melted unsalted butter, packed brown sugar, granulated sugar, and pure vanilla extract until smooth. Add eggs, one at a time, mixing until incorporated.

Add Dry Ingredients: Gently sift in the flour, cornstarch, baking powder, baking soda, and salt. Fold into the wet mixture until just combined.

Incorporate Chocolate Chips: Stir in semi-sweet and dark chocolate chips until evenly distributed throughout the dough.

Preheat and Prepare Baking Sheets: Preheat your oven to 350°F (175°C), and line cookie sheets with parchment paper.

Let the Dough Rest: Allow the dough to rest for about 10 minutes at room temperature.

Scoop the Dough: Using a large cookie scoop, scoop out six tablespoons of dough onto the prepared sheets, spacing them about 2 inches apart.

Bake to Perfection: Bake for 13-15 minutes, until edges are golden brown and centers appear slightly underbaked.

Cool and Serve: Allow cookies to cool on the pan for 5 minutes before transferring to a cooling rack.