The worst "cocktail" I've ever tasted. You better stir that ice because Gin isn't enough to water down the Angostura Bitters.
Now that I think about it, I did with double the volume and sipped. Perhaps it's meant to gulp down as a shot?
Angostura Bitters: Angostura bitters is a concentrated bitters (herbal alcoholic preparation) based on gentian, herbs, and spices, by House of Angostura in Trinidad and Tobago. It is typically used for flavouring beverages or, less often, food. The bitters were first produced in the town of Angostura (now Ciudad Bolívar, Venezuela), hence the name, but do not contain angostura bark. The bottle is recognisable by its distinctive oversized label. Angostura is Spanish for 'narrowing', the town of Angostura having been at the first narrowing of the Orinoco River.