If the style guide you're using doesn't say otherwise, you may omit the comma if the independent clause is sufficiently short.
Here's Kate Turabian on how to use commas in academic writing:
In a sentence containing two or more independent clauses joined by a
coordinating conjunction (and, but, or, nor, for, so, yet), put a
comma before the conjunction. This is not a hard-and-fast rule; no
comma is needed between two short independent clauses with no internal
(Kate L. Turabian, A Manual for Writers of Research Papers, Theses, and Dissertations, Eighth Edition, section 21.2.)
The example of a short enough clause? It's a simple subject / verb / adverbial independent clause.
The senator arrived at noon and the president left at one.
Here's another example from Diana Hacker's A Writer's Reference, 9th edition, P1-b:
The plane took off and we were on our way.
Your own independent clause with a simple direct object ("peanut butter" ) is just as short. Both sources present this as a choice that you may do. You'd have a clear, defensible reason for omitting the comma if you wanted to.
This rule would take precedence over any rule involving a series, since usually guidebooks recommend that you put a comma between three or more items in a list. Not putting a comma between two items in a list is inferred from the omission of such a convention, but it's also a strong enough logical consequence that it feels like a rule. I would wonder too!
So I would focus on whether the sentence is short enough to not need a comma (ex. are the subject and predicate composed of one phrase of three words or less each? if so, omit the comma), and then I'd apply that rule consistently based on what I decided.