Heat the oven to 200°C. Place a round of baking paper in a shallow
Turn down the oven to 150°C. Meanwhile, melt the chocolate in a large bowl over a pan of hot water on a low heat. Melt the butter and allow to cool. Whisk the eggs and sugar together with an electric whisk until pale and very frothy.
Remove the bowl of melted chocolate from the heat and stir in the butter a trickle at a time until it is all incorporated.
Give the egg and sugar mixture a final whisk. Fold it into the chocolate, a spoonful at a time. Spoon the mixture into the tart base, spread it to the edges and bake for 10 minutes. Allow to cool and serve with a dollop of creme fraiche. It can be served tepid.