Preheat your oven to 175°C (350°F) and line your 8×8-inch metal pan with parchment paper, leaving an overhang on two sides for easy removal.
In a large saucepan over low heat, melt the butter and chopped dark chocolate together, stirring constantly until completely smooth. Remove from heat and allow it to cool slightly for five minutes.
Whisk the sugar into the chocolate mixture until combined.
Add the eggs one at a time, whisking well after each addition until the batter looks glossy and cohesive. Stir in the vanilla extract.
Sift the flour, cocoa powder, and salt directly into the pan. Use your spatula to fold the dry ingredients into the chocolate mixture until just combined; do not over-mix, as this will lead to a tough brownie.
Fold in the chopped nuts, reserving a small handful to sprinkle on top if desired.
Pour the batter into the prepared baking pan, spreading it evenly into the corners.
Bake for 25 to 30 minutes. The brownies are ready when the edges are set and a toothpick inserted into the center comes out with a few moist, fudgy crumbs attached, but no wet batter.
Allow the brownies to cool completely in the pan on a wire rack before lifting them out using the parchment paper handles.