4tablespoons (½stick) unsalted butter, at room temperature
Powdered unsweetened cocoa
Boil the cream in a small heavy pan over medium heat until reduced to 2 tablespoons, about 5 minutes. Remove from the heat, stir in the Grand Marnier and chocolate, and return to low heat; stir until the chocolate melts.
Whisk in the butter. When the mixture is smooth, pour into a shallow bowl and refrigerate until firm, 40 minutes.
Scoop the chocolate up with a teaspoon and shape into 1-inch balls. Roll the truffle balls in the unsweetened cocoa.
Store the truffles, covered, in the refrigerator. Let the truffles stand at room temperature for 30 minutes before serving.