Preheat the oven to 350°F. Line 2 cookie sheets with parchment paper. Using beaters or the paddle attachment on a stand mixer, cream the butter and sugars on medium speed until light and fluffy, about 2 minutes.
Add the egg, egg yolk, vanilla extract, and espresso powder, beat on medium speed just until combined, do not over mix. Scraping down the bowl as needed.
Whisk together the flour, cocoa powder, baking soda, cornstarch, and salt. Add it to to the butter and sugar mixture in 2 batches
Mix on medium speed until combined and a dough forms. Scrape down the bowl as needed. Fold in the chocolate chips.
Using a medium cookie scoop, portion out 6 balls of dough per sheet, with enough space to spread in between. Or portion out 60 gram balls. Roll them in your hands making the balls smooth. I recommend making them the right size for the correct texture.
Bake for 10-12 minutes, they will look under baked but will continue baking on the cookie sheet, do not over bake. Scoot the cookies- place a bowl or round cookie cutter that’s larger than your cookie and push the sides up while scooting in a circular motion. Finish with flaky sea salt.