STIR bourbon with three ice cubes (dry to the touch well-frozen ice, not wet ice) in a glass. Add rye whiskey, sugar syrup, bitters and two more ice cubes. STIR some more and add another two ice cubes. And if you have room, add more ice before serving.
Orange zest twist
The melting and stirring in of ice cubes is essential to the dilution, chilling and so the taste of this sublime classic.