Preheat the oven to 200°C/gas mark 6. Lightly grease the pudding moulds and dust with the cocoa powder, discarding any excess. Gently heat the chocolate and butter together in a heavy-bottomed medium saucepan until melted and smooth. Stir in the sugar, then whisk in one whole egg plus one egg yolk and the vanilla extract. Add in the flour, stirring until you have a smooth, thick mixture.
Place the moulds on a baking tray, then fill with the mixture. Cook on the middle shelf of the oven for 15 minutes.
When the cakes are ready, remove from the oven and run a knife around the inside edge of the moulds before turning out on to serving plates. Serve immediately with a sauce of your choice or cream.