In a saucepan, heat the cream and sugar together, stirring, until sugar has dissolved. Let cool a bit.
Beat the butter until light. Gradually beat in the cream mixture - it may seem rough at first but it will become creamy.
Place the mixture in a piping bag. Make 3cm long truffles on a baking sheet lined with baking paper. Place in the freezer and chill at least 45 minutes, overnight is even better. Insert a toothpick into the end of each for the next step.
Melt the chocolate in a double boiler over boiling water or in the microwave. Take the truffles out of the freezer a few at a time and dip in the chocolate, holding the toothpick. Roll in cocoa powder and place in the fridge until eating.