For the cranberry simple syrup: Combine cranberries, sugar, and water in a medium saucepan and bring to a simmer over medium high heat. Reduce heat to medium-low and cook until cranberries are tender but haven't burst, about 10 to 12 minutes. Let cool, then strain through a fine-mesh sieve.
For the cocktail: Combine the rye whiskey, pear liqueur, cranberry syrup, and a dash of bitters in a cocktail shaker. Add ice to fill the shaker two thirds of the way, close, and shake until the shaker is frosted. Strain into a glass filled with ice, top with a wedge of mandarin, and serve.