1. Preheat the oven to 220 C. Butter a 20cm springform cake tin with removable base or 5 small ramekins or oven proof vessels.
2. Place the chocolate, butter and half the sugar in a heatproof bowl over a pan of simmering water and gently melt, stirring occasionally until smooth.
3. Put the eggs, salt and remaining tablespoon of white sugar in a food processor and leave it on for 10 minutes on high speed until the mixture has increased in volume several times. This can be done with a stand mixer or a hand mixer.The mixture will be very thick and turn a pale yellow but almost white when it's ready.
4. Transfer egg and sugar mixture to a large mixing bowl and carefully fold in the flour, then gently mix through the chocolate mixture.If the chocolate is a high percentage the mixture might appear split. If so just add 2 tablespoons of water and whisk until it looks smooth again. The mixture will look quite runny until it starts to cool.
5. Pour the mixture into the cake tin or ramekins and cook for 6 minutes.The rim should have just set while the centre will appear liquid.
6. Leave the cake to cool, then cover and leave in a cool place, but not the fridge overnight. If making individual portions just dust with cocoa and enjoy straight away if you want to eat it warm.