Preheat oven to 350 degrees Fahrenheit and line a large baking sheet with parchment paper or grease lightly.
Beat butter and sugars with an electric mixer on medium speed until pale and creamy, 2-3 minutes. Beat in eggs and vanilla.
In a separate bowl, whisk together flour, cocoa, cornstarch, baking soda and salt. Mix into butter mixture until well combined, then stir in chocolate chips.
Drop tablespoons about 2 inches apart on prepared baking sheet.
Bake until just set and middles no longer look glossy, 10-12 minutes. Let cool on cookie sheet for 2 minutes before transferring to cooling racks to cool completely.
Store in an airtight container at room temperature for up to 5 days, or freeze for longer storage.
Recipe by Hidden Ponies at https://hiddenponies.com/2015/04/triple-chocolate-cookies/