01 - Preheat the oven to 350°F (180°C). Line a 9-inch square baking pan with parchment paper, ensuring an overhang for easy removal.
02 - In a heatproof bowl set over a saucepan of gently simmering water, melt the dark chocolate and unsalted butter, stirring until smooth. Remove from heat and allow to cool slightly.
03 - Whisk in the granulated sugar and light brown sugar until thoroughly combined.
04 - Add the large eggs, one at a time, whisking vigorously after each addition until the mixture is glossy and smooth.
05 - Sift in the all-purpose flour, unsweetened cocoa powder, and fine sea salt. Gently fold with a spatula until no streaks of flour remain.
06 - Carefully fold in the chopped walnuts or pecans and/or dark or milk chocolate chips, if using.
07 - Pour the prepared batter into the lined baking pan and smooth the top evenly.
08 - Bake for 28–32 minutes, or until the top is set and a toothpick inserted into the center emerges with a few moist crumbs.
09 - Allow the brownies to cool completely within the pan before lifting out and cutting into individual squares.