1
Preheat your oven to 180°C (160°C fan). Line a 20x20cm baking tin with baking parchment.
2
Break the chocolate into pieces and chop the butter into cubes. Place both in a heatproof bowl.
3
Melt the chocolate and butter together over a pan of simmering water, stirring until smooth. Remove from heat and let cool slightly.
4
In a large bowl, whisk together the eggs and caster sugar until pale and fluffy. This should take about 3-4 minutes.
5
Gradually pour the melted chocolate mixture into the egg mixture, whisking constantly to combine.
6
Sift in the flour, cocoa powder, and salt, then fold in gently with a large metal spoon until just combined.
7
Stir in the vanilla extract and chopped walnuts (if using). Be careful not to overmix.
8
Pour the mixture into the prepared tin and smooth the top with a spatula.
9
Bake for 25-30 minutes, or until the top is crackly and a skewer inserted into the centre comes out with a few moist crumbs.
10
Leave to cool completely in the tin before cutting into squares and serving.