Bread and butter ______ to be provided.

"Bread and butter" is treated as a single combined item, so it takes a singular verb — therefore use "has."

If you mean the two things separately (bread and butter considered as two distinct items), use the plural verb "have."

• As a single dish (correct here): Bread and butter has been provided.

• If referring to two separate items: Bread and butter have been provided.

Conclusion: Use "has" in this sentence because the phrase refers to one combined item.