In a large, heavy-bottomed saucepan, heat the cream over medium heat until it just begins to simmer, about 5 minutes. Add the chocolate. With a wooden spoon, stir until the chocolate melts and the mixture is smooth, about 5 minutes. Add the butter and stir to melt completely.
If you wish to flavor the mixture, add the Grand Marnier at this point.
Line a baking pan with parchment or waxed paper. Pour the chocolate into the pan, spreading it evenly with a wooden spoon. Let it cool completely, then refrigerate until it sets.
Remove the chocolate from the parchment paper, break it into pieces, and put it in the bowl of an electric mixer fitted with a wire whisk. Beat at low speed for 1 minute, then increase the speed to medium and beat until it is creamy and smooth, 2 to 3 minutes.
Transfer the mixture to a pastry bag without a tip. Pipe out 1/2-ounce truffles, each about the size of a walnut, onto a sheet of parchment or waxed paper.
Combine the cocoa powder and sugar in a shallow bowl. Using two forks, roll the truffles in the mixture, coating them evenly. Transfer to a sheet of parchment paper and leave to harden at room temperature or refrigerate. Store the truffles between layers of parchment paper in an airtight container. Will keep up to 1 week.
An alternative is to dip the truffles in melted chocolate. After piping out the truffles, refrigerate for at least 4 hours. Use dipping chocolate (called couverture), which is available in bittersweet, semisweet, white, or milk chocolate varieties in specialty food shops. It melts easily and smoothly. If you plan to dip all of the truffles, you will need about 2 pounds of chocolate, but if you want to do only 2 to 3 dozen, you will need about a pound. Work in a cool, dry room. Melt the chocolate in a stainless-steel bowl set over simmering water. Use a four-prong fork to dip, then tip the truffles onto a sheet of parchment paper by turning the fork upside down. Avoid touching the dipped candies while they set. Chocolate is shinier if allowed to cool at room temperature. Refrigerate the candies only if the chocolate has not set after 15 minutes. After they set, put them in paper candy cups and store between sheets of parchment paper.
Yield: 80 Truffles