"Never drink more than two Pan Galactic Gargle Blasters unless you are a thirty-ton mega elephant with bronchial pneumonia." - Zaphod Beeblebrox
Douglas Adams has confirmed making one of these is impossible on Earth (in part due to the laws of physics). This hasn't stopped fans from making many different versions of the recipe, leading to many heated arguments about what it should look or taste like or how true it should be to the original recipe.
The first recipe featured here is a slight adaptation of the PGGB that's often served in geek-themed bars, while the second is slightly more true to the books. The original, impossible instructions are as follows.
♦︎ The Hitchhiker's Guide to the Galaxy ♦︎
♦︎ Cocktail Chemistry Lab ♦︎ Christos Chouseas (Bar 42) ♦︎ io9 ♦︎ GeekyChef ♦︎ SilverMoonDragon on AllRecipes ♦︎ Wikibooks ♦︎