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Pour the port into a small, heavy saucepan. Stick the cloves in the strips of lemon zest and add them to the port with the cinnamon stick and sugar. Gradually bring to a boil over moderate heat and boil the mixture until slightly reduced and just beginning to be syrupy, 3 to 5 minutes. Dissolve the cornstarch in the lemon juice and gradually stir it into the Port Sauce. Reduce the heat and simmer for 1 to 2 minutes; the sauce will thicken. Strain the sauce into a heatproof bowl and let cool to room temperature.
Meanwhile, using a metal skewer, make a starter hole in the center of the skin side of each plum quarter. Skewer two plum quarters on each cinnamon stick, skin side to cut side, with a strip of lemon zest between them.
Set up your grill for direct grilling and preheat the grill to high. Brush and oil the grill grate. Arrange the plum kebabs on the grate. Grill until the plums are golden brown, lightly basting with some of the port sauce, 3 to 5 minutes per side.
Meanwhile, scoop vanilla ice cream into 4 large martini glasses or wine goblets. Top with the plum kebabs and any remaining port sauce. Garnish with the mint sprigs and serve at once.
The recipe can be prepared through Step 2 several hours ahead.