Preheat your oven to 350°F and position a rack in the center. Line two baking sheets with parchment paper to prevent sticking.
In a medium mixing bowl, whisk together the flour, sifted cocoa powder, cornstarch, baking soda, and salt. Set this aside for now.
Using a stand mixer with the paddle attachment (or a handheld mixer), beat the softened butter, light brown sugar, and granulated sugar on medium speed until the mixture is light and fluffy, about 2 minutes.
Crack in the egg and pour in the vanilla extract. Beat until just combined.
Gradually add the dry ingredients from your bowl, mixing on low speed to avoid making a mess. Continue until fully blended.
Remove the bowl from the mixer and gently fold in the chocolate chips with a rubber spatula.
Scoop out cookie dough using a 1/4-cup measuring spoon or cookie scoop. Place them on the prepared baking sheets, spacing them a few inches apart to allow for spreading.
Bake each sheet one at a time for 12 minutes. The cookies will look slightly underdone but will firm up as they cool.
Let the cookies cool on the baking sheet for 10 minutes, then transfer them to a wire rack to cool completely. This part requires patience, but it’s worth it!