There’s not much history we can find regarding the Monte Carlo, but for a brief mention in David Embury’s The Fine Art of Mixing Drinks, where it’s presented in a very sweet 2:1 ratio, shaken, with no garnish. Sam Ross and the folks at Milk + Honey have a lovely app of their own called Bartender’s Choice which specifies a 4:1 ratio we much prefer, states it should be stirred, not shaken (of course!), and grants it the missing piece: a garnish of lemon peel, expressed to mist the oils atop the drink.