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Temper chocolate, add cream in thirds, add cold butter and whisk a while. Chill, then scoop balls and chill them again. Temper another batch of chocolate, dump truffles in it and roll between your gloved hands (like this you'll get a very thin layer of coating). Toss into powder, then sieve exzess off.
My current favourite is using a fine olive oil as flavour, covering them in white chocolate and tossing them in 抹茶 powder. I find cocoa a bit obvious. I tried also きな粉, but that doesn't add much. The 抹茶 adds very little taste, so it really needn't be the highest grade, but it looks very nice.