Preheat the oven to 180°C and line a square baking pan with parchment paper, leaving an overhang on two sides for easy removal.

Melt the butter and chopped dark chocolate together in a heatproof bowl over a pan of simmering water, stirring occasionally, until smooth and glossy.

In a separate bowl, whisk the sugar, eggs, and vanilla extract vigorously for 3 minutes until the mixture turns pale and slightly thickened.

Pour the melted chocolate mixture into the whisked egg and sugar mixture, folding gently until fully combined and glossy.

Sift the flour, cocoa powder, and salt over the wet mixture and fold gently with a spatula just until no dry streaks remain.

Gently fold in the chocolate chips and chopped walnuts, distributing them evenly through the batter.

Pour the batter into the prepared baking pan and spread it evenly to the corners with a spatula.

Bake for 28 minutes until the top looks set and crackly but a toothpick inserted in the center still comes out with a few moist crumbs.

Let the brownies cool in the pan for 15 minutes so they set properly and are easier to slice cleanly.

Lift the brownies out using the parchment overhang, place on a cutting board, and slice into 9 even squares.