In reply to:

What's Espanish for 'chopped liver'?

Paté........

Bueno. Hígado picado.

It's not so much that the b and v are interchangeable, the v most often sounds like b, although there are also instances in which the b sounds more like v, ie bacha. Vaca, incidently, is one of the words most likely to be spelled "baca".