Preheat oven to 350F (180C). Butter three 6oz moulds or ramekins, dust with cocoa powder and place in the fridge for about 10 minutes.
Chop the chocolate and place in a heatproof bowl, add butter. Set the bowl over a double boiler (pot with simmering water). Melt the chocolate. Remove from heat and let cool slightly.
In a separate bowl whisk eggs, sugar and salt, until light, pale and thick.
Slowly pour melted chocolate into the egg mixture.
Sift in flout and whisk just until incorporated.
Pour into prepared molds. Fill about 80%.
Bake for 10-11 minutes.
You can eat it straight from the mold or invert onto a plate, dust with powdered sugar and serve.