Fill a mixing glass halfway with ice. Add bourbon, limoncello, and simple syrup.
Stir gently for about 20 seconds until chilled.
Add bitters and give it another quick stir.
Strain into an Old Fashioned glass over fresh ice.
Express lemon peel over the glass, drop it in, and garnish with mint or rosemary if desired.
Notes
Use bourbon for mellow warmth, or rye for a spicy kick. Chill limoncello for best results. Swap simple syrup for honey syrup if you prefer. Fresh herbs add aroma without overpowering the drink.