Combine all the ingredients in a bowl and set it above a saucepan of water. Allow the water to come to the boil, then turn the heat down to low. Stir the chocolate mixture occasionally until completely melted and smooth. Remove the bowl from the heat.
You can either let the chocolate cool down in the bowl you melted it in, or for quicker cooling, scrape it into a second wide bowl. Transfer to the refrigerator for a few hours to set firmly.
Put some extra cacao powder in a bowl to use for dusting the truffles. Take the bowl of chocolate out of the fridge and use a teaspoon to scoop out a bite-sized amount of mixture, dropping it into the bowl of cacao. Toss it in the cacao, then use your fingers to squish the chocolate into a roughly round shape. Toss it in the cacao again, and this time roll it briefly in the palms of your hands to make a smooth ball. Place on a plate and continue making truffles with the rest of the mixture. Store the truffles in the refrigerator.