In a small saucepan, bring the cream and orange peel just to a simmer over medium heat (watch carefully so the mixture does not boil over). Remove and let stand for 15 minutes.
Place the chocolate in a medium heat-proof bowl with a fine mesh strainer set on top. Reheat the cream mixture to a simmer. Add the orange liqueur, if using, to the cream and stir to combine. Pour the mixture through the strainer onto the chocolate, pressing gently on the orange peel to extract all of the liquid; discard peel. Let chocolate mixture stand for 1 minute. Slowly whisk the chocolate mixture until smooth. Cool to room temperature. Cover the bowl and chill for 1 to 3 hours or until the mixture is firm enough to shape.
Place the cocoa powder in a small, shallow bowl and dust your hands with it. For each truffle, scoop out some of the thickened chocolate mixture using a rounded measuring teaspoon and quickly form it into a rough 1-inch ball. Roll in the cocoa powder; shake off excess cocoa. Arrange on a plate. Repeat with remaining mixture, rechilling as necessary. Refrigerate truffles until ready to serve. Store in an airtight container in the refrigerator up to 1 week. Let stand at room temperature about 30 minutes before serving.