There are fudgy brownies, and then there are these: extra fudgy, supremely chocolaty, and so dark they’re almost black. This is an indulgent, adult brownie, with no flour to mute the darkest chocolate flavor (and so, as a bonus, it satisfies our gluten-free friends). Flourless brownies can taste like undercooked batter, however. To create a baked good, not a confection, we add just 3 tablespoons of cornstarch; it binds the ingredients, resulting in brownies with a melt-in-your-mouth center and a crispy, chewy edge. For dark chocolate flavor with dimension, we include a mix of chocolate—bittersweet, unsweetened, and a generous 1/4 cup of black cocoa powder that makes our brownies deeper and darker than your average. The cocoa powder also works, along with the cornstarch, to prevent our brownies from being pasty or wet. Finally, we achieve a shiny, sleek, obsidian-like finish with an unadulteratedly chocolate water ganache, made from bittersweet chocolate and more black cocoa powder, that gently drips down the sides.
We prefer black cocoa powder in this recipe. You can substitute regular Dutch-processed cocoa powder for the black cocoa, but the brownies will not be as dark and the ganache will turn out thicker. – America’s Test Kitchen