To make the brownies:
1. Adjust oven rack to lowest position and heat oven to 350 degrees. Make foil sling by folding 2 long sheets of aluminum foil so that they are as wide as 13 by 9-inch baking pan (one 13-inch sheet and one 9-inch sheet). Lay sheets of foil in pan perpendicular to one another, with extra foil hanging over edges of pan. Push foil into corners and up sides of pan, smoothing foil flush to pan. Grease foil and set aside.
2. Whisk cocoa, espresso powder, if using, and boiling water together in large bowl until smooth. Add unsweetened chocolate and whisk until chocolate is melted. Whisk in oil and melted butter. (Mixture may look curdled.) Add eggs, yolks, and vanilla and continue to whisk until smooth and homogeneous. Whisk in sugar until fully incorporated. Whisk together flour and salt in small bowl and then mix into batter with rubber spatula until combined. Fold in bittersweet chocolate pieces.
3. Transfer batter to prepared pan; spread batter into corners of pan and smooth surface. Bake until toothpick inserted in center of brownies comes out with few moist crumbs attached, 30 to 35 minutes, rotating pan halfway through baking. Transfer pan to wire rack and let cool for 11/2 hours.
4. Remove brownies from pan using foil. Return brownies to wire rack and let cool completely, about 1 hour. Cut brownies into 2-inch squares and serve. (Brownies can be stored in airtight container at room temperature for up to 3 days.)
Why this recipe works
Brownies are a tricky business; homemade recipes have better flavor, while boxed mixes claim best texture. Our goal was clear: a homemade brownie with chewiness (and a shiny, crisp, crackly top) to rival the boxed-mix standard, but flush with a rich, deep, all-natural chocolate flavor. What it all comes down to is the ratio of saturated to unsaturated fats.
Go for chew: Most homemade brownies are either super-rich and fudgy or more cakelike and less rich. The difference between the two styles is simply the ratio of chocolate to flour. Chewy brownies, like the kind you get from a boxed mix, are very hard to make from scratch. We tried all kinds of tricks — everything from using condensed milk and biscuit mix instead of flour to cooking the sugar and butter into a caramel — but nothing worked. We even replaced the granulated sugar with brown sugar, but that only caused us to lose the shiny, crackly top that we expect from good brownies.
Look to the box: It turns out that the whole key to the texture of a box brownie resides in the specific types and amounts of fat it includes. To get boxed-mix brownie chew in homemade, we discovered the perfect proportion of liquid to solid fat, without the aid of high-tech fats used by brownie mix makers. It all came down to the magic 1:3 ratio of saturated to unsaturated fat (as opposed to the 2:1 ratio found in homemade brownies made with butter). We balance the saturated fat of butter with unsaturated vegetable oil. To simplify the calculations in our version we chose to eliminate melted chocolate and use cocoa powder, which contains very little fat by comparison.
Add extra yolks: To reduce greasiness, we first tried reducing the overall fat content but found that left the brownies dry. We remembered that emulsifiers can help prevent fats from separating and leaking out during baking. We tried using mayonnaise, which worked surprisingly well, producing brownies with a rich, luxurious texture. But when we dove deeper, we identified the active emulsifier in the mayonnaise as lecithin, a phospholipid that occurs naturally in egg yolks. Rather than using mayonnaise, the simple addition of two extra yolks in exchange for a little oil made greasiness a thing of the past.
Bloom the cocoa: At this point, our recipe relied on just cocoa. We realized we could replace some of the butter in our recipe with unsweetened chocolate without changing the ratio of fats. We poured boiling water over the cocoa to bloom its flavor and added a little instant espresso for intensity. We then whisked in the chocolate until melted.
End with chunks of chocolate: Only chocolate that is melted and incorporated into the batter actually affects the ratio of fats in the mix. Therefore, for even more chocolate flavor, we incorporate chocolate chunks into the mixed batter. Because they don’t melt until the batter starts baking, they have no effect on texture. The final results are as close to a boxed-mix texture as any home cook could produce without the benefits of industrial processing: chewy, fudgy bars with gooey pockets of melted chocolate that evoke images of bake sales past, but with complex flavor and just enough adult flourish to lift them out of the realm of child’s fare.
Blooming cocoa in hot water — or hot coffee — is the key to bringing out the deep chocolate flavor of this concentrated source of cocoa solids. Cocoa powder is an important ingredient, both for its flavor and for the fact that it contains little fat, which can negatively affect the texture of chewy brownies or sturdy cupcakes.
For an accurate measurement of boiling water, bring a full kettle of water to a boil, then measure out the desired amount. For the chewiest texture, it is important to let the brownies cool thoroughly before cutting. If your baking dish is glass, let the brownies cool for 10 minutes, then remove them promptly from the pan (otherwise, the superior heat retention of glass can lead to overbaking). Use high-quality chocolate in this recipe.