The cocoa powder tends to become damp if the truffles are left for much longer than a couple of hours. For an alternative finish which will store better, cut up 110g dark chocolate. Place in a bowl over a pan of simmering water to melt. Do not allow the chocolate to get too hot. Remove from the heat. When it is cool but still runny, place the cold truffles into the chocolate, remove with a fork then roll on a plate covered with grated chocolate. Place straight into petit four cases.