What is a common component in both the Sidecar and the Adonis cocktails?

The correct answer identifies vermouth as a common component in both the Sidecar and the Adonis cocktails. In the Sidecar, although the primary spirit is cognac, the cocktail also incorporates a sweet element which can often come from orange liqueur. The Adonis, on the other hand, is primarily composed of sweet vermouth combined with sherry. Both cocktails utilize vermouth to enhance flavor complexity, offering herbal and aromatic notes that complement the other ingredients.

Vermouth serves not just as a filler but as a key flavor component that balances the sweetness and acidity in cocktails, making it a staple in various classic drinks. This commonality highlights the versatility of vermouth in cocktail preparation, demonstrating how it successfully bridges different base spirits and styles.