Preheat the oven to 180 C/350 F. Line an 8 x 8-inch pan with parchment paper and set aside.: As the oven warms, you should feel the kitchen shift from ambient to cozy, and the air will smell faintly of warm metal. A properly preheated oven gives even rise and sets the edges while keeping the center fudgy. Using parchment ensures easy removal and clean edges. A common slip is not letting the oven fully reach temperature, which leads to underbaked centers even when the top looks done.
In a small bowl, sift your cocoa powder and cornstarch to avoid any clumps in the batter. Add the salt.: Sifting produces a light, airy powder that blends seamlessly, eliminating grainy pockets of cocoa powder or cornstarch . Run your fingers through the sifted mix to confirm it is uniform and fine. This step helps the batter become smooth quickly. If you skip sifting, you may detect small dry flecks in the finished brownies.
In a small saucepan, add your butter and chocolate. On low heat, heat the butter and chocolate until melted and glossy. Remove from heat.: Watch the mixture as it melts, stirring gently; you want a glossy, ribbon like texture without steam or bubbling. The smell will turn deeply chocolatey and warm, signaling readiness. Removing from heat right away prevents overheating, which can cause the chocolate to seize or become grainy. Overcooking here is the main reason a batter loses its silkiness.
Moving quickly, add your sugar and whisk very well until combined. Add the eggs, one at a time, and mix well into the mixture. Add your cornstarch, cocoa powder, and salt, and mix until the batter is smooth and no longer grainy. Fold through your walnuts.: After the glossy chocolate base is off heat, the sugar dissolves into the warmth, smoothing texture and contributing to that crackly top. Adding eggs one at a time ensures they incorporate evenly, creating a cohesive batter. When you fold in the sifted dry mix, watch for a uniform, lustrous batter without streaks. Finally, folding in the walnuts by hand preserves their identity and crunch. A common error is whipping vigorously at the egg stage, which can trap too much air and create a cake like brownie.
Transfer your brownie mixture to the lined pan. Top with extra walnuts and bake for 26-28 minutes, or until a toothpick comes out just clean from the center.: Spoon the batter into the prepared pan and smooth the surface; you should see a glossy, slightly thick surface before it goes into the oven. Nestling a few extra walnuts on top creates a toasty finish. As it bakes you may hear the faintest crackle and smell rich chocolate; the edges will set first and darken slightly. The toothpick test should return just clean with a few moist crumbs for fudgy brownies. Leaving it in too long will yield a drier, cakier result.
Remove from the oven and let the brownies cool completely before slicing.: Cooling is the final, quiet stage where the brownies firm up and set into neat squares. The aroma mellows and the interior finishes its texture transformation. If you try to slice them warm, they will smudge and fall apart; patience yields clean cuts and attractive portions. A frequent impatience is cutting too soon, which ruins presentation and makes serving messy.