Preheat the oven. Preheat to 350°F (175°C). Line an 8-inch or 9-inch square baking pan with parchment paper, leaving an overhang on two sides for easy removal.
Melt the butter. In a medium saucepan over medium heat, melt the butter until it just begins to bubble.
Add sugar and cocoa. Remove from heat and whisk in sugar and cocoa powder until the mixture resembles wet sand. This blooms the cocoa and deepens the chocolate flavor.
Cool slightly. Let the mixture cool for 3 to 5 minutes so it doesn't scramble the eggs when added.
Whisk the eggs. In a separate bowl, whisk eggs with sugar vigorously for 60 to 90 seconds until the mixture becomes foamy, pale, and thick. This step is critical for achieving the shiny crackly top.
Combine. Pour the warm chocolate mixture into the egg mixture and whisk together for 20 to 30 seconds until glossy and slightly tightened.
Add vanilla and salt. Stir in vanilla extract and salt.
Fold in flour. Sift in the flour and stir vigorously for 30 to 45 seconds until the batter is thick, glossy, and free of lumps. Do not overmix.
Add optional mix-ins. Fold in chocolate chips or nuts if using.
Pour and spread. Pour batter into the prepared pan and spread evenly with a spatula.
Bake.
💠8-inch pan: bake 20 to 25 minutes
💠9-inch pan: bake 30 to 40 minutes
💠Brownies are done when the edges are set but the center still looks slightly underdone. A toothpick inserted in the center should come out with moist crumbs, not wet batter.
Cool completely. Remove from oven and cool in the pan for at least 30 minutes before cutting. Brownies continue cooking as they cool and become fudgier as they rest.
Slice and serve. Use a hot, dry knife and wipe the blade between each cut for clean slices.
