The Hernandez is a clever low-proof riff on a Manhattan, subbing in vanilla liqueur like Navan or Tuaca for the bourbon or rye. That simple substitution brings out the best of the supporting ingredients, allowing a complex array of vanilla, fig and spice flavors to shine through.
Your vermouth should be very, very fresh for this drink. Vermouth begins to oxidize a couple weeks after it’s opened, imparting a bitter tinfoil-esque flavor.