In a stand mixer, blend melted butter, granulated sugar, light brown sugar, and vanilla extract for 1 minute on medium speed.
Add eggs and mix on low until combined.
Incorporate flour, corn starch, cocoa powder, baking soda, and salt, mixing on low until everything comes together.
Using a medium-sized cookie dough scoop (about 2-2.5 tablespoons), form 20 even cookie balls on a parchment-lined baking sheet.
Refrigerate for 30 minutes to enhance flavor and prevent spreading. Preheat the oven to 350°F.
Space chilled cookie dough 2 inches apart on a parchment-lined sheet and bake for 9-10 minutes until edges are set.
Cool on the baking sheet for 3 minutes, then transfer to a rack to cool completely.