Who would have thought that gin, lemon, and chocolate would go together so well? It sounds like a weird combination, but give it a shot and you'll be hooked. The first time you take a sip of a Twentieth Century you'll be a little confused. You're not sure if it's sweet or not, if its tart or boozy, you get a hint of chocolate but also the botanicals from your gin.
All the ingredients come together harmoniously to create a truly unique drink. It's a good way to use your bottle of creme de cacao, especially if you're trying to appeal to people who don't want something like a Brandy Alexander. A lot of recipes will call for white creme de cacao to keep the drink a pale yellow color, but I prefer to sacrifice aesthetics for flavor by using a higher quality creme de cacao.
If you're unable to find Lillet Blanc, I would suggest using something like Cocchi Americano, Dolin Blanc, or another good quality sweet white vermouth.