Place heavy cream in a large pot and bring slowly to a boil over medium heat. Turn off the heat and stir in the chopped chocolate. Let stand for 2 minutes then stir well until smooth. Stir in the butter. Pour into the bowl of an electric mixer, and let cool until set.
Fit mixer with a paddle and beat the chocolate mixture on medium speed until aerated and fluffy, about 2 minutes. Beat in the liqueur and/or other flavorings. Load ganache into a pastry bag fitted with a half inch plain tube. Line several baking sheet with parchment or waxed paper. Pipe the truffle mixture out to form 1/2-inch balls. Place pans in the refrigerator to chill thoroughly.
To coat the truffles, set up your counter like this: tray of chilled centers on your left, bowl of melted tempered chocolate in the middle, large pan filled with cocoa powder on your right. (Reverse this if you're left handed) Pick up a truffle with your left hand. Touch the palm of your right hand lightly onto surface of tempered chocolate. Drop the truffle into your chocolaty right hand and roll it around to coat. Move your right hand over the pan of cocoa powder, turn your hand over, and drop the coated truffle into the cocoa. Repeat the process to coat about 5 truffles, then pick up a fork with your clean left hand and use it to roll the truffles round a bit in the cacao. (A second person can help do this continuously.) Repeat until the cocoa pan is filled with a lot of truffles. Then wash and dry your hands (or enlist your helper to do this); place the truffles in a strainer and shake it over the pan of cocoa powder to dislodge any excess cocoa. Place finished truffles on another paper-lined pan to set completely. Truffles do not need to be refrigerated.
Repeat coating processes with remaining centers.
c.1997, M.S. Milliken & S. Feniger, all rights reserved