Place the chocolate in a small, heatproof bowl. Place the cream, Grand Marnier, and zest in a small saucepan and bring to a simmer, stirring to combine. Pour the hot cream mixture over the chocolate and let sit for 2 minutes and then whisk until smooth.
Pour the melted chocolate mixture into an 8×8-inch baking dish and let cool in the refrigerator for half an hour or so until completely chilled.
Place the pistachios on a plate. Line a baking sheet with parchment paper. Scrape out one spoonful of chocolate and roll into a ball. Roll the chocolate balls in the chopped pistachios and place on the parchment-lined baking sheet. Repeat until all the chocolate and pistachios are used up. (NOTE: Your hands will get pretty messy as the chocolate softens – you will probably have to stop and wash your hands a few times during the process. Also, if the chocolate gets too soft, pop it back in the refrigerator for 10-15 minutes to harden a bit.)
Store the truffles in an airtight container in the refrigerator until ready to eat.