Buttermilk does not contain butter or milk. But what it is is basically the leftover when you churn butter out of milk.
It is used as an analogy by Ramana Maharshi to illustrate the relationship between Brahman, Eeshvara and Jagat.
From Sayings of Sri Ramakrishna (848):
As long as you are in the plane of relativity, you must admit both 'butter' and 'buttermilk' ,- you must admit both Personal God (Ishvara) and the universe, To explain the analogy, the original milk is Brahman (Nirguna) realized in Samadhi, the 'butter' the Impersonal-personal God (Saguna), and the 'buttermilk' the universe made up of the twenty-four categories.