Divide the simple syrup, liqueur, rye and bitters between 2 lowball glasses. Mix well. Add 1 large ice cube to each glass. Squeeze the orange zest over the glasses and then add to the cocktails. Serve immediately.
Bring the sugar and water to a boil in a small saucepan; cook about 10 minutes. Cool and use immediately or store, covered, in a refrigerator.