Melt the chocolate and cream together in 20-second blasts in the microwave or in a glass bowl set over a small pan of simmering water. Stir in the vanilla, then pour into a small bowl. Once cool, put in the fridge to chill for 2-3 hours. When firm, use a melon baller or 2 teaspoons to scoop out and form balls, then roll each truffle in either the cocoa powder or the chopped pistachios. Chill the leftover bits to use them up. Keep chilled until ready to serve.