Fudgy Triple-Chocolate Brownies

1. Adjust oven rack to lower-middle 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 8-inch square baking pan. 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. Melt chocolates and butter in medium heatproof bowl set over saucepan of barely simmering water, stirring occasionally, until smooth. Whisk in cocoa until smooth. Set aside to cool slightly.
3. Whisk eggs, sugar, vanilla, and salt together in medium bowl until combined, about 15 seconds. Whisk warm chocolate mixture into egg mixture. Using rubber spatula, stir in flour until just combined. Transfer batter to prepared pan; spread batter into corners of pan and smooth surface. Bake until slightly puffed and toothpick inserted in center of brownies comes out with few moist crumbs attached, 35 to 40 minutes, rotating pan halfway through baking. Let brownies cool in pan on wire rack to room temperature, about 2 hours. Remove brownies from pan using foil. Cut brownies into 1-inch squares and serve. (Do not cut brownies until ready to serve. Brownies can be wrapped in plastic wrap and refrigerated for up to 3 days.)

TRIPLE-CHOCOLATE ESPRESSO BROWNIES
Whisk in 1-1⁄2 tablespoons instant espresso powder or instant coffee along with cocoa in step 2.