verb agreement - Bread and butter is/are my breakfast?

Bread and butter can be singular or plural, depending on context.

In British English and various European languages (i.e. German Butterbrot, Russian Бутерброд), "bread and butter" is a set phrase meaning "an open-face sandwich".
This phrase is not common in American English; we would regard "bread and butter" as referring to two separate items and therefore plural.

In any case, it's odd to say that something "is my breakfast" if I am in the very act of eating it; it may be a grammatically valid sentence, but no native speaker would say it (unless - as @PeteKirkham pointed out - s/he had a bread-and-butter fetish!) It would be more natural to say "I'm having bread and butter" if I'm eating it right now; "I had bread and butter for breakfast" if I'm speaking of some time in the past, or "bread and butter is my usual breakfast" if I eat it regularly.

Bread and butter is also an idiomatic phrase.

• As a noun, it means "a dependable source of income": "Fixing flat tires is my bread and butter."


• As a hyphenated adjective, it means "basic or essential": "Job creation is the Senator's bread-and-butter issue."