Heat the cream and butter in a small saucepan until the butter melts and the mixture is simmering. Place the chocolate in a heatproof bowl and pour over the hot cream. Add the chilli powder and stir together until the chocolate melts. Chill for at least 4 hours until firm.
Using a melon baller or lightly oiled hands, shape the chocolate mixture into small walnut-sized balls. Spread the cocoa powder on a plate, then roll each chocolate ball in the cocoa until well coated.
Transfer to individual paper cases or arrange on a plate and serve, sprinkled with a little extra chilli powder, if liked. The truffles will keep for up to 3 days in the refrigerator.