First prepare the salted caramel sauce so it has time to cool down. Place the sugar and water in a medium heavy bottomed saucepan over medium-high heat, and cook without stirring until sugar dissolves and gets a caramel color.
Remove the pan from the heat and add the cream. It will bubble a little bit.
Cook for about 1 minute stirring constantly with a wooden spoon until smooth.
Remove from heat and add vanilla extract, butter and salt. Stir well until smooth. Transfer into a bowl and set aside to cool.
Preheat the oven to 400°F (200°C).
Butter 4 ramekins 6-8 fl oz (170-230ml) capacity and dust with flour or cocoa powder.
Melt chocolate and butter over a bain-marie until smooth. Meanwhile beat the eggs with sugar and salt using a whisk or mixer until thick and light yellow. Mix in vanilla extract and flour. Gradually add the melted chocolate and stir well. Pour ¾ of the mixture into the prepared ramekins.
Add 1 heaping teaspoon of caramel into the center of each.
Cover with remaining batter.
Bake for about 14-15 mins until the edges are firm and the center is still soft. Let cool for 3-4 minutes.
Run a sharp knife completely around the inside edge of the molds and invert onto plates. Top with salted caramel sauce and fleur de sel, and serve immediately.