You see Angostura bitters used all the time in cocktails, but you very rarely see it as the main ingredient. The Angostura Sour uses a whopping 1.5 oz of Angostura Bitters as the main spirit in the cocktail. Keep in mind, it's 89 Proof (44.7% ABV), so it's actually stronger than a lot of spirits people drink regularly.
If you've ever tasted the bitters before, you know how strong the flavor is. That intensity comes through in the drink but is mellowed by the egg white and ounce of simple syrup. The notes of cinnamon, orange peel, and clove from the bitters shine through and provide a nice alternative to a classic sour formula. I love this drink in the winter (and in the summer), or anytime I want something a spicy flavor.