Grease six small ovenproof moulds (each containing approx. 150 ml) and sprinkle with sugar. Chop the chocolate and melt with 100 g butter in a hot water bath at low heat. Preheat the oven (electric cooker: 175 °C/circulating air: 150 °C/gas: s.
Beat the eggs, egg yolks and 60 g sugar with the whisks of the mixer for about 5 minutes until creamy. Stir in flour and chocolate mix. Fill the moulds to approx. 3⁄4 with the mixture. Bake in a hot oven for 10-12 minutes.
The core should still be liquid.
Meanwhile, for the sauce, select the blackberries, wash and drain them if necessary. Caramelise 1 tablespoon of sugar in a pot. Add the blackberries. Heat for 1-2 minutes while stirring carefully until the caramel has dissolved.
Pour the liqueur. Pour into a tall mixing bowl and puree finely with a hand blender. Pass the blackberry-cassis sauce through a sieve. Divide the sauce between six plates. Take the fondant au chocolat out of the oven and turn it out onto the blackberry-cassis sauce.
Dust with icing sugar and serve immediately.