Butter the sides of a heavy 2-quart saucepan. In the saucepan combine sugar, evaporated milk, and the 1/2 cup butter. Cook and stir over medium-high heat until mixture comes to boiling. Reduce heat to medium; cook for 6 minutes more, stirring constantly. Remove from heat.
Add marshmallows, dark chocolate, Grand Marnier liqueur, and orange peel; stir until melted and smooth. Stir in dried cranberries. Beat by hand for 1 minute. Transfer mixture to a large bowl. Cover and chill about 1 1/2 hours or until nearly firm.
Dip balls, one at a time, into melted chocolate, letting excess drip back into bowl. Return chocolate-dipped balls to cookie sheet. Chill about 30 minutes or until chocolate is set. If desired, sprinkle truffles lightly with cocoa powder before serving.