Finely chop 300g of dark chocolate (70% cocoa) and place in a heatproof bowl.
Heat 300ml of heavy cream and 50g of unsalted butter until the butter melts and the cream just simmers. Pour over the chocolate and stir until smooth.
Cool, then refrigerate for at least 4 hours, until firm.
Scoop small portions with a teaspoon. Roll into balls with lightly oiled hands, then roll in unsweetened cocoa powder. Keep refrigerated until serving.
