Break the chocolate into small pieces and melt it with cream and rum over hot water.
Whisk gently until the mixture is smooth.
Let it cool in the refrigerator overnight covered by plastic wrap.
Scoop a small tablespoon of chocolate mixture and roll the truffle balls in your hands; be careful to avoid melting the balls. The size (diameter) should be about 1 inch (2.5 cm). Alternatively, shape the truffles by using a melon baller.
Roll the balls in cocoa powder.
Store the truffle balls tightly covered in a refrigerator.
These truffles can also be covered by dark chocolate: first melt the chocolate in a double boiler, cool to 95 degrees F (35 degrees C) or less, then roll cold truffle balls in the melted chocolate using a fork and a teaspoon, finally put the truffles in the refrigerator for at least 30 minutes. Chocolate-covered truffles should be served at room temperature.