In a small saucepan, heat heavy cream with orange zest over medium heat just until boiling point. Place the chocolate chips in a medium bowl and pour the hot cream over it. Let it melt undisturbed for 2 minutes, and then slowly mix with a spatula until smooth. Stir in Grand Marnier and cardamom.
Pour the chocolate mixture into a shallow dish (I used 8-in (20cm) square baking pan) and chill for at least 2 hours in the fridge, until set.
Using a medium cookie scoop, form the chocolate mixture into 1.5-inch (about 3.5cm) balls. If desired, roll the truffles in cocoa powder. Keep refrigerated for up to 5 days.