The River Cafe is in London and is where Jamie Oliver trained, plus many others. I am not a huge chocolate person, but this is a complete uncomplicated chocolate experience.
I am hoping you have a metric scale as this is a British recipe, hence the grams.

Ingredients

 675 grams really good dark chocolate , 70%
450 grams butter
10 whole eggs, at room temperature
675 grams caster sugar

Method

Melt the chocolate and butter together in a bain marie. (This means put it into a bowl over simmering water, but don’t let the bowl touch the water.)
Remove from the heat and allow to cool
Preheat the oven to 320F and line a 27 cm springform cake tin with foil.

Beat together the eggs and sugar with an electric mixer for 5 to 8 minutes until they’ve quadrupled in volume.
Fold the chocolate and butter mixture into the eggs and sugar and stir thoroughly.
Pour the mixture into the prepared cake tin. You might have a little too much batter but don’t worry.
Place the cake in a large roasting tin in the oven then pour enough boiling water in the roasting tin to come 3/4 up the side of the cake.
Cook in the preheated oven for over an hour, then turn the oven off and allow to completely cool before removing.
Serve with creme fraiche and raspberries.