Break the chocolate into pieces and melt it into the water bath with the butter. Separate the egg whites from the yolks. Beat the egg yolks to foam with the sugar, beat the whites to snow with a pinch of salt. Mix the yolk mixture with the chocolate one, stir and carefully add the egg white mixture.
Pour the mixture into a round form that you've laid out with baking paper. Bake the cake in a preheated 356°F (180 °C) oven for about 30 min.
While still warm, sprinkle with cocoa, powdered sugar or wait for it to cool, then glaze with sour cream or buttercream.