1 Heat the oven to 160C fan / 180C / 350F / gas 4. Toast the walnuts in a dry skillet until fragrant. Remove and let cool.
2 Slit the vanilla bean lengthways and scrape out the seeds. Place the seeds and the bean pod in a pan with the milk and bring to a boil. Melt the chocolate over simmering water.
3 Melt the butter in a pan, stir in the flour and cook briefly to make a roux.
4 Remove the vanilla bean pod from the milk and stir the milk into the roux. Simmer gently for 10 minutes, stirring occasionally.
5 Pour the mixture into a large bowl, let cool slightly and stir in 2 egg whites. Add the egg yolks one by one, stirring the mixture to a smooth consistency each time. Add the melted chocolate and nuts.
6 Beat the remaining egg whites and sugar until stiff and whisk about ¼ into the chocolate mixture. Fold the rest of the egg whites in carefully using a spatula.
7 Butter the ramekins and scatter with sugar. Pipe the cake batter into the ramekins using a piping bag with a large nozzle. Fill the ramekins only ¾ full (the cakes will rise in the oven).
8 Place the ramekins in a bain-marie and bake in the oven for around 35 minutes (checking with a skewer to see if cooked through).
9 Remove from the oven and, whilst still hot, pour over melted chocolate. Leave to cool and serve.