Which ingredient gives the Old 43 cocktail a hint of vanilla?

The ingredient that provides a hint of vanilla in the Old 43 cocktail is Woodford bourbon. Bourbon, particularly Woodford Reserve, is a type of whiskey that often has notes of vanilla due to the way it is aged in charred oak barrels. The aging process extracts flavors from the wood, which includes sweet vanilla notes, thus contributing complexity and depth to the cocktail.

While simple syrup, vanilla extract, and citrus flavors can all bring different taste elements to a drink, it is the bourbon's unique profile that naturally includes those vanilla characteristics. This makes it the correct choice for identifying the ingredient responsible for the hint of vanilla flavor in the Old 43 cocktail.