Add 2 oz bourbon (or rye whiskey) to a mixing glass
Add 1/4 to 1/2 oz simple syrup (to taste)
Add 2 dashes bitters (Angostura or similar)
Add 1 dash orange bitters (optional)
Add ice to the mixing glass
Stir 20 to 30 seconds until well-chilled
Strain into a rocks glass over a large ice cube
Garnish with an orange twist and/or a brandied cherry (optional)
For a stirred version: stir and strain as above
For a “build in glass” version: add syrup and bitters to a rocks glass, add bourbon, add ice, stir briefly, and garnish