Chop the chocolate and melt it with the butter in a small pot while stirring at low heat. Beat the eggs, egg yolks and 60 g sugar with the whisk of the hand mixer for about 5 minutes until creamy.
Stir in flour and chocolate.
Spread the dough into the greased and sugar-coated hollows of a muffin tray (12 hollows each). Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for 10-12 minutes.
In the meantime, beat the remaining sugar and sour cream with the whisk of the hand mixer for about 3 minutes until stiff. Remove from the oven, remove from the mould, arrange with the sour cream and dust with cocoa.