To unlock: Know pi to >79 digits.
Effect: Everything is pi times as efficient. Golden Cookies' effects last pi times longer and appear pi times as often.
Flavor text: "Some things are deceivingly helpful."
Trick to using: it gives you back what you spend, so your bank will be unaffected, but it counts as cookie production, so it can give you a major boost in "Cookies baked (this ascension)" and can be useful for getting those "Bake X cookies" achievements.