Make a bain-marie: place a heatproof bowl over a pan of barely simmering water, ensuring the bowl doesn't touch the water. Add the chocolate to the bowl and stir till melted and smooth, then remove from the heat.
Bring the cream to the boil in a heavy saucepan. Remove from the heat, add the melted chocolate, and stir well. Refrigerate at least 6 hours, or overnight.
After chilling, place the bowl over a pan of barely simmering water yet again. Add the butter and mix well to get a smooth paste. Remove from the heat and let cool for about an hour.
When set, remove portions of the chocolate mixture and roll into small balls, then roll in the cocoa powder. Set aside on greaseproof paper. Store in the fridge till needed.