Step 1Heat the barbecue grill or grill pan to medium heat.
Step 2Combine the sugar and mint leaves in a mortar and pestle or spice grinder and grind to a paste. Divide the mixture in half.
Step 3Cut each plum into 6 wedges and discard the pits. Divide the plums between two 12-inch square pieces of foil. Sprinkle half of the mint sugar over the plums. Fold up the foil sides and carefully pour the Cointreau over to coat the plums, then bring up the sides of the foil to form two leakproof packets. Plum foil packets can be made ahead of time and refrigerated until ready to grill. Remove them from the refrigerator 15 minutes before grilling.
Step 4Place the packets on the grill and cook until the plums are knife tender, about 4 to 6 minutes depending on the grill. Do not overcook. Divide into 4 bowls and sprinkle with some of the remaining sugar, saving the rest for another use. Serve immediately.