This one actually comes with a proper name ("The Greenway Sour") but it's in the same notepad as the others, so it counts as a Notebook Cocktail. It may be one of my favorites.
1 oz white rum (Plantation 3 Star, duh)
1 oz London Dry gin (Beefeater in this case, but my love of Boodles is well known)
3/4 oz rhubarb syrup (a slightly spiced version with nutmeg and lemon peel)
3/4 oz lime juice
Shake briefly and strain over ice, then top with about 1 1/2 oz charged cucumber water (an excellent mixer and a fine use for those left-over cucumber solids).