Chewy and Fudgy Homemade Brownies

Preheat oven to 325 degrees F. Line an 8x8 baking pan with parchment paper. Set aside.
Add butter and vegetable oil to a small saucepan over low heat. Heat until the butter is fully melted, then add the dark chocolate, sugar, brown sugar, cocoa powder, and espresso powder. Continue to cook until the chocolate is fully melted, then whisk together for 1-2 minutes, until all ingredients are incorporated and the sugar begins to dissolve.
In a separate large bowl, combine eggs, egg yolk, and vanilla extract. Use a hand mixer to mix everything together on medium speed for 2-3 minutes, until the eggs are bubbly, frothy, and a pale yellow color. This is what gives the brownies a crackly top.
Pour the melted chocolate mixture into the egg mixture and mix on low until combined, about 30 seconds.
Then add the all purpose, cornstarch, and kosher salt. Continue to fold together. Fold the chocolate chips into the batter.
Then pour the brownie batter into the prepared pan. Bake for 40-45 minutes, until the edges are set and the center is slightly jiggly, akin to jello when shaken.
Allow the brownies to cool to room temperature for 30 minutes, then place into the refrigerator to set for 2 hours. Cut with a sharp knife into 12 brownies and top with flaky sea salt.