In a large saucepan, combine the sugar, wine, water, orange and lemon zests, cinnamon stick and vanilla bean and bring to a boil, stirring to dissolve the sugar. Set the pears in the syrup, stems up, and simmer over moderately low heat, setting them upright if they roll around, until tender when pierced with a knife, about 20 minutes. Using a slotted spoon, transfer the pears to a large dish to cool.
Strain the syrup into a medium saucepan. Boil over moderate heat until the syrup thickens to the consistency of maple syrup and registers 220° on a candy thermometer, about 10 minutes. Let cool to room temperature, then refrigerate until chilled.
In a small bowl, whisk 1/4 cup of the reduced syrup into the mascarpone and chill. Using an apple corer or teaspoon, core the pears all the way through.
In a large bowl, beat the heavy cream until stiff peaks form. Fold in the chilled mascarpone. Spoon the cream into a pastry bag fitted with a medium star or plain tip. Stand the pears on a platter and pipe the mascarpone cream into their centers.
Spoon a heaping tablespoon of the Bittersweet Chocolate Sauce onto each of 8 dessert plates. Set the pears on the sauce and spoon a little more chocolate sauce over each pear. Pipe the remaining mascarpone cream around the pears and serve.
The cream-stuffed poached pears can be prepared through Step 4 and refrigerated for up to 1 day; refrigerate the remaining mascarpone cream in the pastry bag.