The classic Trinidad Sour uses 1½ oz Angostura bitters. First, who wants to pry the dasher cap off the bitters bottle. Second, ounce for ounce, bitters are much more expensive than a typical 750ml bottle of liquor. This variation on the classic uses Amaro di Angostura instead. The bonus is that you now also have a delicious bottle of amaro to sip on later.