Soak pistachios overnight to soften, especially if not using a high speed blender. Pistachios are fairly soft without soaking, but this will yield a smoother and fluffier sauce.
Place ingredients into a high speed blender or food processor and blend until smooth. Douse everything you eat with it. It's amazing!
You can also use roasted pistachios but you may get a slightly different taste. Make sure they are unsalted though as the cayenne pepper sauce has plenty of salt and it will be much too salty with both.