Preheat the oven to 350°F. Line two large baking sheets with parchment paper and set aside.
In the bowl of a stand mixer fitted with the paddle attachment, cream together the softened butter, granulated sugar and light brown sugar for 1-2 minutes until light and fluffy. Add the eggs, vanilla extract and whole milk. Mix until combined, scraping down the sides of the bowl as needed.
In a separate bowl, whisk together the flour, cocoa powder, cornstarch, baking powder, and salt. Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Be careful not to over mix.
Using a medium 2-tablespoon cookie scoop, portion the dough onto the prepared baking sheets, spacing the cookies approximately 2 inches apart. Do not flatten the dough balls.
Bake for approximately 9–11 minutes, or until the edges are set and the centers still appear soft.
mmediately after removing the cookies from the oven, use a round cookie cutter or glass to gently reshape them into perfectly round cookies, if desired. Allow the cookies to cool on the baking sheet for 3–5 minutes before transferring to a wire rack to cool completely.