Preheat your oven to 180°C (350°F) and line a 20x20 cm (8x8 inch) baking pan with parchment paper, leaving some overhang for easy removal.
Place the dark chocolate and butter in a heatproof bowl and melt them together over simmering water, stirring gently until smooth and glossy. Let the mixture cool slightly.
Pour the melted chocolate and butter into a mixing bowl, then add the sugar and whisk until combined and shiny.
Add the eggs one at a time to the mixture, whisking well after each addition until fully incorporated and the batter is smooth.
Sift in the plain flour and salt, then gently fold them into the batter using a spatula until just combined. Add the chopped walnuts and fold briefly.
Pour the batter into the prepared baking pan, spreading it evenly with the spatula to ensure a smooth surface.
Bake in the preheated oven for 20-25 minutes, until the edges are dark and slightly crackly while the center remains just set and slightly gooey.
Remove the brownies from the oven and let them cool in the pan for about 10 minutes, then transfer them to a wire rack using the parchment overhang.
Allow the brownies to cool completely before slicing into squares to reveal a rich, fudgy interior with a crackly top.
