In a medium bowl, stir together the flour, cocoa, cornstarch, baking soda, and salt and set aside.
In a large bowl, using an electric mixer fitted with a paddle or whisk attachment, cream the butter, sugar, and brown sugar on medium-high speed for about 2-3 minutes. Then add the egg and vanilla and mix for about 1 minute, until creamy and well combined. Add the flour mixture and mix on medium-low speed just until incorporated, about 30-60 seconds. Then stir in the chocolate chips, if used, just until evenly distributed. Cover and chill for 1 hour.
Preheat the oven to 350°F (175°C). Line two or three baking sheets with parchment paper and set aside.
Scoop 2 tablespoons (45 g) of cookie dough per cookie, roll into even balls with the palm of your hands and place 6 cookies, spaced 3 to 4 inches (7.5 to 10 cm) apart, on each prepared baking sheet. Bake one sheet at a time for 10-11 minutes, or until edges are set, and the centers start to get small rips on top and look dry and puffy.
Remove from the oven and let cool for about 5 minutes on the baking sheet, or until firm enough to move. They will deflate as they cool. Then, transfer to a wire rack and let cool completely. The cookies will stay fresh in an airtight container at room temperature for up to 4 days or freeze for up to 3 months.