Whisk the sugar, rum, vanilla, orange zest and 1/4 teaspoon salt in a large bowl until the sugar dissolves. Add the cherries and cornstarch and toss; set aside until juicy, about 30 minutes. Line a baking sheet with foil and coat with cooking spray. Place six 5-to-6-inch ceramic tart dishes on the prepared pan. Add 3/4 cup cherries to each dish using a slotted spoon; top each with 1 tablespoon butter and 2 teaspoons chocolate. Transfer the remaining cherry mixture to a saucepan; add 1/2 cup water and simmer over medium heat, stirring occasionally, until thickened, 5 to 7 minutes. Divide among the dishes; cool completely.
Dust a surface with flour. Using a paring knife, cut out 6 circles from the piecrust, 1/2 inch bigger than the tart dishes (use the dishes as a guide). Place on another baking sheet; chill until ready to bake. Position a rack in the lower third of the oven and preheat to 375 degrees F. Top each dish with a piece of dough, pressing the edge against the dish to seal; pierce several times with a knife. Bake 45 minutes, then brush each pie with heavy cream and sprinkle with sugar. Continue baking until golden, about 15 more minutes. Serve with ice cream.