Roughly chop the chocolate and melt over a warm water bath. Mix the butter in pieces, eggs, flour, sugar and melted chocolate with the whisk of the hand mixer to a smooth dough.
Pour into 4 ramequins (each with a capacity of 125 ml). Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 15 minutes, remove from oven. Let the cake rest for about 10 minutes, then carefully turn it over.
Ice cream or whipped cream tastes good with it.