In a mixing bowl, whisk together the all-purpose flour, dark cocoa powder, cornstarch, baking soda, baking powder, and salt. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment or in a large mixing bowl with an electric hand mixer, mix together the melted unsalted butter, granulated sugar, and light brown sugar until well combined. Add in the eggs and vanilla extract and mix until smooth.
Add in the flour mixture and mix on low speed until the cookie dough forms. Scrape the sides and bottom of the bowl to ensure the ingredients are well combined.
Use a large cookie scoop (3 tablespoons) to portion out the cookie dough. Roll the cookie dough into smooth balls and place on a parchment-lined baking tray. Chill the cookie dough balls for 30 minutes.
Preheat the oven to 350℉ (177℃). Line baking sheet pans with silicone baking mats or parchment paper.
Place the chilled cookie dough balls on the prepared baking sheet pans, allowing room for spreading. Bake the cookies for about 11 minutes until the edges are set and the centers are puffed up. Cool the cookies on the pan for 2 minutes and then transfer the cookies to a wire rack to cool completely.