Grind 1-1/2 lbs. of semisweet chocolate (that have already been coarsely chopped) in an electric food processor.
In a heavy saucepan, boil 1/2 cup of heavy cream and 1/2 cup of buttermilk or sour cream. Slowly pour this créme fraiche into the ground chocolate mixture.
Mix until smooth and chill in the refrigerator for about 15 minutes. Then, blend in 1/2 stick (or six tablespoons) of softened, unsalted butter and 3 tablespoons of cognac. Mix thoroughly and refrigerate for another few hours.
Use an ice-cream scoop to shape the truffle balls and place them on the baking sheet. Dip the chocolate balls (a toothpick can come in handy here!) in the remaining chocolate, which can be melted with some vegetable oil. Refrigerate the truffles for hours until they harden.
Some like to imprint their truffles with a gold leaf. All you need to do is to gently press a quarter-sized sheet of gold leaf onto each truffle. This is best done right after the truffle has been dipped in for its chocolate coating.