Q. What exactly is the "liqueur" called for in truffle recipes? What is in it? Where can I buy it?
Truffle recipes offer a lot of latitude in flavorings. Generally you can mix a couple tablespoons or more of some flavoring agent into the truffle mixture (generally a combination of chocolate and cream, and/or butter, egg yolks, etc.). In your case, the recipe writer is letting you make the decision.
Among liqueurs, it is common to add amaretto for almond-flavored truffles, Frangelico for hazelnut truffles, Nocino or Crème de Noix for walnut flavor, Cointreau or Grand Marnier for an orange flavor, Fraise des Bois or Chambord for raspberry flavor, Kahlua or another coffee liqueur, an apricot brandy liqueur, another fruit brandy liqueur, or various melon liqueurs, or Poire William for a very subtle pear flavor. Various crème liqueurs are often used as well, including crème de cacao (to intensify the chocolate taste), crème de banane, crème de cassis (blackcurrant), crème de fraise (strawberry), crème de framboise (another raspberry option), and the ever-popular crème de menthe (mint).
Truffles may also be flavored or fortified with various spirits, probably the most common of which is rum. But cognac and other brandies, Calvados (apple), and Kirsch (cherry) also show up in some recipes we've seen.
Needless to say, you do not have to use any alcohol to flavor truffles. Fruit, mint, and spice oils, extracts, and zest can impart amazing flavor. We make a cinnamon truffle that the whole world is waiting to taste (it just doesn't know it yet). And to our taste buds, there is nothing at all wrong with a plain chocolate truffle either.