Make the Brownie Batter: In a large mixing bowl, add the eggs, egg yolk, and granulated sugar. Whisk for about 1-2 minutes, until the sugar is fully dissolved. (Feel the mixture between your fingers - if it's completely smooth, you're good! If you feel sugar granules, keep whisking.) Next, whisk in the powdered sugar, vanilla, and salt. Then, check that the chocolate mixture is no longer piping hot (it will be quite warm, though), and whisk it into the bowl of eggs/sugar/. Fold in the flour & espresso powder, then the chocolate chips. The brownie batter won't be very thick.
3 large eggs, at room temperature , 1 large egg yolk, at room temperature , 3/4 cup granulated sugar , 1 and 1/2 cups powdered sugar , 2 tsp vanilla extract , 1/4 tsp salt, 1/2 cup all-purpose flour , pinch of espresso powder, 1 cup semi-sweet chocolate chips