Pre-heat the oven to 160°C.
Lightly butter a 25cm springform cake tin then line the base with a circle of baking parchment and the edge with a long strip. Break the chocolate into a large Pyrex bowl. Dice and add the butter. Sit the bowl over a pan of simmering water, making sure that the bowl does not touch the water.
Leave the chocolate and butter to melt, stirring occasionally until the mix becomes smooth and glossy. Make sure the chocolate doesn't get too hot or it will seize and thicken. When ready, turn off the heat, but keep the bowl sitting on the pan to keep the mix warm.
Spearate your eggs by breaking the egg carefull in two, passing the yolk gently between two parts of the shell, catching the falling white in a bowl. Be careful not to break or damage the yolks, as even a tiny amount of yolk in the whites will prevent them from whisking up properly. Put the yolks into a bowl with the sugar and whisk to the ribbon stage (the mixture will become pale and thick and leave a trail on the surface for a few seconds). This will take 4-5 minutes. When ready, gently fold the yolks into the melted chocolate.
Next, take a large stainless steel bowl and whisk the egg whites into soft peaks - the tips of the peaks should just fold over, not stand upright. Fold the whites into the chocolate mix.
Pour into the prepared tin, place in the oven and bake for 45 minutes. It will soufflé up during the cooking and start to crack on the surface when it's ready. When you take it out the oven, the cake will collapse in the middle but don't panic! This is just the way it should be. Leave in the tin to cool.
When cool, carefully remove the cake from the tin by placing the palm of your hand over the cake and inverting it out then peel off the paper. Flip back onto a board. Dust with icing sugar, cut the cake into wedges and serve with a dollop of crème fraîche and a heavy drizzle of caramel sauce.