Preheat oven to 180°C (160°C Fan/350°F). Line an 8x8-inch square baking tin with parchment paper.
Melt the dark chocolate and butter together in a heatproof bowl set over a pan of simmering water. Stir until smooth, then set aside to cool slightly.
In a large bowl, whisk the eggs, egg yolk, and caster sugar together for 3-5 minutes until thick, pale, and leaving a trailing ribbon.
Gently fold the cooled melted chocolate mixture into the whipped eggs using a spatula.
Sift the flour, cocoa powder, and salt over the wet mixture. Fold gently until just combined and no dry flour remains.
Pour batter into the prepared tin. Bake for 25-30 minutes until the edges are set but the center has a slight wobble.
Allow to cool completely in the tin, then chill in the fridge before slicing for the best fudgy texture.
