There are several tasty options. I usually go with unsweetened cocoa powder, making sure to use Dutch-processed cocoa powder which is milder and less acidic. Other options include unsweetened shredded coconut, PB2 (powdered peanut butter), and finely chopped nuts. You can also coat these truffles in almond flour - basically the same as chopped nuts, but a finer grind, so it's easier to work with.