Preheat oven to 180°C/350°F/Gas Mark 4. Lightly butter six 250 ml (8 fl oz) ovenproof ramekins. Coat with caster sugar and tip out the excess. Make a bain-marie by placing a roasting dish on a shelf in the middle of the oven and pouring in boiling water to a depth of 3 cm (1 ¼ in).
Put butter, chocolate and half the sugar in a heatproof bowl and melt over a saucepan of simmering water. Sieve flour and cocoa and mix with breadcrumbs and hazelnuts. Beat egg whites with remaining sugar until firm peaks form. Mix egg yolks and vanilla into chocolate mixture, then stir in flour mixture. Stir one-third of meringue into chocolate mixture then carefully fold in remaining two-thirds until just combined. Spoon mixture into prepared dishes and place them in the bain-marie. Bake 20-30 minutes. Test with a thin knife or skewer pushed into the middle – it should come out clean. Remove bain-marie carefully from the oven and then remove puddings in their ramekins using tongs or carefully using your fingers. Leave for a few minutes before turning out. Serve straight away with either whipped cream or mascarpone. They can also be successfully reheated later in a microwave.