01 - Preheat the oven to 350°F. Line an 8-inch square baking pan with parchment paper, leaving an overhang on two sides for easy removal.
02 - In a large bowl, whisk together the all-purpose flour, unsweetened cocoa powder, and salt until evenly distributed.
03 - In a separate bowl, combine the melted butter, granulated sugar, brown sugar, and vanilla extract, stirring until smooth and no sugar lumps remain.
04 - Add the eggs one at a time, mixing thoroughly after each addition until the mixture is glossy and well emulsified.
05 - Gently fold the dry ingredient mixture into the wet mixture using a rubber spatula, stirring just until combined. Avoid overmixing to keep the texture tender.
06 - Fold in the chopped dark chocolate and nuts if using, reserving a small handful of each for the topping.
07 - Pour the batter into the prepared baking pan and spread it evenly with a spatula. Scatter the reserved chocolate and nuts over the top.
08 - Bake for 28 to 32 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs but not wet batter.
09 - Allow the brownies to cool completely in the pan on a wire rack. Once fully cooled, use the parchment overhang to lift them out and cut into 16 even squares.