In a large mixing bowl, whisk together flour, cocoa powder, espresso powder, cornstarch, baking soda and sea salt. Set aside.
In a separate bowl, beat butter, brown sugar and granulated sugar with an electric mixer until creamy and combined.
Beat in egg and vanilla until light and fluffy, scraping down the sides of the bowl as needed.
Reduce mixer speed to low then gradually add dry ingredients to wet. Mix until just combined, being careful to avoid overmixing.
Cover bowl with plastic wrap and chill in the refrigerator for a minimum of 3 hours.
Remove dough from the refrigerator and allow to rest at room temperature for 30 minutes.
Preheat oven to 350 degrees. Line 2 large baking sheets with parchment paper and set aside.
Form dough into 1-inch balls and place on prepared baking sheets. Gently flatten dough with your fingertips then bake for 8 minutes. Cookies are done when the edges are set and centers are still slightly soft. Avoid overbaking.
Cool cookies on baking sheets for 5 minutes then transfer to a baking rack to cool completely.