Grease an 8 inch tin with butter
Line the base with greaseproof paper and dust the edges with cocoa powder
Preheat the oven to 170°C
Place chocolate and butter in a bowl and melt over a pan of gently simmering water
In a separate bowl, whisk eggs and sugars until combined
Pour in the melted chocolate then add the vanilla and salt
Sift in the flour and fold in until just combined
Transfer to the tin and bake for 18-22 minutes
Remove when the edges are set and there is a slight wobble in the centre
Allow to stand for 5-10 minutes before removing from the tin and serving
