Preheat oven to 400 degrees. Simmer port and 1/2 cup brown sugar in a saucepan over medium heat until reduced to 1/2 cup, about 25 minutes. Transfer to a bowl. Cover, and let cool 10 minutes.
On a lightly floured surface, roll out dough to 1/8-inch thickness. Fit into an 8-inch pie dish, leaving a 1-inch overhang.
Stir together remaining 3/4 cup brown sugar and remaining 1/2 teaspoon salt, plums, cornstarch, cinnamon, and port syrup. Transfer to pie shell. Fold in overhang to form a crust; brush crust with cream, and sprinkle with sanding sugar. Bake 30 minutes; reduce oven to 375 degrees. Bake until golden and center is bubbling, about 45 minutes more. Let cool.