Heat oven to 325ºF (163ºC). Lightly grease an 8✕8-inch square metal baking pan with olive oil. Line pan with parchment paper, leaving a 2-inch overhang on 2 sides for easy removal of brownies; brush parchment with olive oil.
In a medium bowl, whisk together flour, cornstarch, salt and baking soda.
In a large bowl, whisk together sugar, oil, eggs, yolk, and vanilla until smooth. Add cocoa powder and espresso powder and vigorously whisk until smooth and glossy, about 1 minute.
Add flour mixture to cocoa mixture and mix, using a rubber spatula, until mostly combined and a few pockets of dry flour remain.
Fold in chocolate chunks.
Scrape batter into prepared pan and smooth top. Bake brownies until set, firm to touch and a toothpick inserted in the center comes out with a few sticky crumbs, 30–35 minutes.
Transfer pan to a wire rack and immediately sprinkle with flaky sea salt, if using. Let brownies cool in pan 15 minutes, then remove brownies from pan by lifting parchment overhang; transfer to wire rack to cool completely.
Cut into 9 squares.