Combine the cream, butter, and salt in a saucepan over medium heat and cook until the butter is melted and the mixture just comes to a boil. (Watch carefully -- it can boil over quickly if you aren't paying close attention.) As soon as it comes to a boil, pour the hot cream mixture over the chocolate pieces in a heatproof bowl. Allow it to sit, undisturbed, for 1 to 2 minutes. Whisk until completely smooth. Cool to room temperature, stirring frequently, then stir in the Grand Marnier and vanilla.

When the truffle mixture begins to thicken, cover it and place in the refrigerator, stirring occasionally, until thoroughly chilled and firm.

Scoop the mixture into tablespoon-size portions and roll into smooth balls. Roll the balls in the cocoa powder, and transfer to a serving platter or to candy boxes.

Truffle mixture may be made up to one week in advance and then scooped, rolled, and dusted just before serving.