These fudgy brownies are tender, moist, and ultra-chocolatey. They're infinitely better than boxed brownies - you can taste the difference immediately.
Adjust oven rack to lower middle position and preheat to 350F. Line a 9x13 baking pan with long sheets of foil so that excess foil hangs over edges of pan for easy removal later on. Push foil into corners and up sides of pan, smoothing foil against pan. Grease foil generously and set aside.
Whisk cocoa, espresso powder, and boiling water (measure out correct amount from your kettle of boiling water) together in large bowl until smooth. Add unsweetened chocolate and whisk until chocolate is melted. Whisk in oil and melted butter. Add eggs, egg yolks, and vanilla. Continue to whisk until smooth and incorporated. Whisk in sugar until thoroughly incorporated. Sprinkle salt and flour into batter and fold with rubber spatula until just combined. Fold in mini chocolate chips.
Transfer batter into prepared pan evenly. Bake for about 35-40 minutes. It's done when toothpick comes out with moist crumbs attached; should not come out clean. Do not overbake, or brownies may be dry. It will seem underbaked, but will set to fudgy consistency upon cooling.
Transfer pan to wire rack and cool for 1 1/2 hours if pan is metal; 20 minutes if pan is glass. Very gently remove brownies from pan using foil sling. Return brownies to wire rack and let cool completely. Cut brownies into squares; you may have to gently peel foil off bottoms.