1. Preheat the oven to 190°C, gas mark 5.
2. Melt the butter and chopped chocolate in a heatproof bowl set over a saucepan or pot of simmering water. Stir until smooth, then remove from the heat and set aside to cool.
3. In a separate bowl, whisk together the eggs, egg yolks, sugar, vanilla and salt for about 3 minutes, until it has at least doubled in volume, pale and mousse-like – this is more easily achieved using a stand mixer or an electric hand whisk.
4. Using a rubber spatula or large metal spoon, fold the melted chocolate and butter into the egg mixture, being careful not to knock out too much air. Sift in the cocoa powder, flour and ground hazelnuts and gently fold to combine.
5. Half fill each of the prepared ramekins with the fondant mixture and place a teaspoon of chocolate sauce in the middle of the fondant mixture. Divide the remaining fondant mixture between the ramekins, filling each one almost to the top but leaving a gap of about ½ cm.
6. Arrange the ramekins on a large baking sheet and cook on the middle shelf of the preheated oven for 10–12 minutes. The fondants should be well-risen and still soft in the middle.
7. Remove from the oven and set aside to rest in the ramekins for 30 seconds–1 minute, then run a palette knife around the edges to turn the fondants out onto plates. Serve immediately and watch the middle melt.