Start by placing the sugar cube in an old-fashioned glass (also known as a rocks glass).
Add 2-3 dashes of bitters directly onto the sugar cube.
Muddle the sugar cube with the bitters until it's partially dissolved. If using simple syrup, you can skip this step and directly add the syrup.
Add a few ice cubes to the glass.
Pour the bourbon or rye whiskey over the ice.
Stir the mixture gently for about 30 seconds to combine and chill.
Take a fresh orange peel and express the oils over the drink by holding it over the glass and giving it a gentle twist. Rub the rim of the glass with the peel, then drop it into the drink as a garnish.
Optionally, you can add a cherry as an additional garnish.
