Adjust the oven rack to the lowest position and preheat oven to 350 degrees. Make a foil sling for a 9-inch by 13-inch baking pan. The first sheet should be 13 inches wide and the second sheets should be 9 inches wide. Lay sheets of foil in pan perpendicular to each other with extra foil hanging over the edges of the pan. Push foil into corners and up the sides of the pan, smooth foil flush to pan. Coat foil with nonstick spray.
In a large bowl, whisk together boiling water, cocoa powder, and espresso powder (if using). Whisk in unsweetened chocolate until smooth. Whisk in oil and melted butter (mixture may look curdled).
Add eggs, yolks, and vanilla and whisk until smooth and evenly combined. Whisk in sugar, then whisk in flour and salt until smooth. Stir batter with a rubber spatula (scraping the bottom of the bowl), then fold in bittersweet chocolate.
Pour batter into prepared pan and spread into corners, smoothing the top. Bake until a toothpick inserted comes out clean with a few crumbs attached, about 30 to 35 minutes, rotating pan halfway through baking time.
Transfer baking pan to a wire rack and cool brownies for 1 1/2 hours. Using foil overhang, lift brownies out of the pan. Return brownies to wire rack and cool completely, about 1 hour longer. Cut brownies into 2-inch squares.
Water: The easiest way to measure boiling water is to bring a full kettle to boil, then measure out the amount you need.
Instant espresso powder: Sold in small jars, this instant coffee substitute boosts chocolate flavor in a big way.
Oil and butter: The trick to a chewy brownie is to keep the saturated fat (butter) low, but to raise the unsaturated fat (oil, eggs). We still want a little butter for flavor, but oil in the brownie batter is a one-way ticket to Chewy Town.
Foil sling: Cut two long lengths of foil that match the two sides of the baking pan: 9 inches and 13 inches. Next, place the sheets in the pan overlapping and perpendicular to each other, like a T, letting the extra foil come up over the edges. Press the foil into the corners of the pan, and smooth out the bottom. Then you can grease the foil with nonstick spray. Press the extra foil against the outside of the pan, but don't trim it off. That's what you'll use to gently lift out the brownies once they're completely cool.
Glass pan: If using a glass baking pan, cool the brownies in the pan 10 minutes, but then lift out using the foil sling (the glass pan may continue cooking the brownies when they are out of the oven).
Under-baking: It's better to take the brownies out a little too early rather than a little late. This keeps them moist.
Cool completely: COMPLETELY. You need to wait if you want them to be truly chewy.
Storage: Store brownies in an airtight container at room temperature for up to 3 days.
Freezing: Slice into squares and wrap in foil before freezing. Thaw at room temperature.
Walnuts: Love nuts? Add 3 ounces toasted walnuts in place of half the chocolate chips in Step 3.