Grease a 20 cm (8-inch) springform pan and place a round of parchment paper at the bottom, then flour it.
Melt the chocolate with the butter using a double boiler or in the microwave.
Remove from heat and add salted butter caramel and stir until smooth. Let cool before starting to whisk the eggs. Add it cold to the egg-and-sugar mixture.
Whisk the eggs with the brown sugar and muscovado sugar in a stand mixer on medium speed (4) using the KitchenAid. You can use a hand mixer. This takes about ten minutes, but it is important that the mixture whitens and triples in volume.
Add the melted chocolate and butter along with a pinch of Guérande sea salt (optional), then mix on speed 8 until well combined.
Sift in the flour and whisk well to combine.
Pour into the mould and leave to rest for 30 min at room temperature, then place in the fridge for 1 h 30 before baking.
Note: This resting time allows air bubbles to rise to the surface and form the crunchy meringue crust on top when baked.
Preheat the oven to 200°C (392°F) on fan-assisted mode.
Bake for 5 minutes, then lower the temperature to 120°C (248°F) and extend the baking time for about 35 to 45 minutes. Keep an eye on it starting at 35 minutes.
Note: Cooking time is important to obtain the right texture and depends on each oven. To adjust the baking time for your oven, keep an eye on it starting at 35 minutes. The center should remain slightly wobbly. If you overcook it, you won't get any lava in the fondant.
Remove the cake from the oven and leave to cool for 1 hour before unmolding. The fondant will pull away from the edges of the pan, but you can run a knife or a wooden skewer around the edge to help with unmolding
Leave it at room temperature for at least 8 hours, preferably overnight. So you can enjoy its extra-melting texture. If you wish it a little firmer, place it in the refrigerator.
Serve it cold or warm by reheating it for a few minutes in a preheated and turned-off oven.