In a medium bowl, whisk together 1 ½ cups all-purpose flour, ½ cup unsweetened cocoa powder, 1 teaspoon cornstarch, ¾ teaspoon baking soda, ¼ teaspoon salt to combine. Set aside.
Using a stand mixer or large bowl with a hand mixer on medium speed, add ½ cup butter and mix until smooth. Pour in the ¾ cup brown sugar and continue mixing until fluffy and combined.
Add in the large egg and 1 teaspoon vanilla extract and mix until combined.
Turn the mixer on low speed and slowly add in the flour mixture. Mix the cookie batter just until it is combined, careful not to overmix.
Place the cookie dough on some plastic wrap and pop into the refrigerator for at least 1 hour (until firm) before scooping to bake.
Preheat the oven to 350 degrees F. and line a baking sheet with parchment paper.
Remove the cookie batter from the refrigerator and scoop onto the prepared baking. Slightly dampen your fingers and gently press down in the center to flatten each individual cookie.
Place the baking sheet into the oven and bake for 8 to 10 minutes, until the center of the cookies begins to set. It is best to set the cook timer for 4 to 5 minutes, check the cookies, and adjust the time as needed.
Remove the cookies from the oven and allow them to rest in the baking sheet on a cooling rack for 2 minutes before removing the cookies to cool completely.
Store the cookies in an airtight container at room temperature. Enjoy!