Preheat the oven to 120°C/250°F/gas ½. Snap the chocolate into a heatproof bowl, add the butter, espresso and a good pinch of sea salt, and place over a pan of gently simmering water to melt very slowly until smooth, stirring regularly. Meanwhile, whisk the eggs and sugar together until pale and doubled in size, then carefully fold in the melted chocolate mixture.
Boil the kettle. Divide the mixture between 6 coffee cups or ramekins and put them into a roasting tray. Place the tray in the oven, then carefully pour in enough boiling kettle water to come halfway up the side of the cups. Bake for exactly 20 minutes, then carefully remove from the oven and leave to cool in the water for 2 hours. To serve, I sometimes shave over some extra chocolate, or add fresh fruit like cherries, blood oranges or wild strawberries, with a dollop of yoghurt or crème fraÎche.