4ounces (113 grams) bittersweet chocolate, chopped fine
Instructions
Adjust oven rack to lower-middle position and heat oven to 325°F. Make foil sling for 8‑inch square baking pan by folding 2 long sheets of aluminum foil so each is 8 inches wide. Lay sheets of foil in pan perpendicular to each other, with extra foil hanging over edges of pan. Push foil into corners and up sides of pan, smoothing foil flush to pan. Grease foil.
For the brownies: Whisk cocoa, cornstarch, and salt together in bowl. Microwave butter, bittersweet chocolate, and unsweetened chocolate in large bowl at 50 percent power, stirring occasionally, until melted, 2 to 4 minutes. Whisk in sugar, vanilla, and espresso powder until well combined. Whisk in eggs until fully combined and smooth. Stir in cocoa mixture until just incorporated. Transfer batter to prepared pan and smooth top.
Bake until toothpick inserted in center comes out with few moist crumbs attached, 33 to 37 minutes, rotating pan halfway through baking. Let brownies cool completely in pan on wire rack, about 2 hours. Using foil overhang, remove brownies from pan.
For the ganache: Bring water, cocoa, and corn syrup to simmer in small saucepan over medium heat, whisking to dissolve cocoa, then immediately remove from heat. Whisk in chocolate until melted and smooth. Pour ganache over brownies, spreading with offset spatula to cover top (ganache may drip over side but does not have to cover sides). Let sit until ganache is set, about 1 hour. Before serving, using hot knife, cut brownies into 9 squares, then cut each square in half diagonally to create 2 triangles, dipping knife in very hot water and wiping knife clean and dry between each cut.