When made to Lucas Bols original specification: 1 shot Bols Genever, 1 shot Bols Butterscotch liqueur and ¾ shot sweet vermouth, this drink has a great flavour but lacks balls (or perhaps I should say Bols). Hence the formula above which is heavy on the Genever with equal parts butterscotch liqueur and sweet vermouth. It's even better when an Oude Genever is used rather than a Jonge as this amplifies the butterscotch notes.
Beware, Bols' original recipe calls for a lemon zest twist garnish but the lemon oils overpower the butterscotch, so if you use a slither of lemon skin to fix our choice of a Werther's Original to the rim, then don't express the lemon's oils over the drink.
Don't be tempted to use buttercup flowers as a garnish as they are poisonous. They are also poisonous when eaten by livestock but their acrid taste and the blistering of the mouth they cause mean that animals don't tend to eat them. Interestingly, the toxins degrade when dried, so hay containing dried buttercups is safe for animals to eat.
My (Simon Difford's) adaptation of a recipe I discovered in January 2014 at The House of Bols, Amsterdam, Netherlands.