It's a tricky construction, one that has bothered writers for eons. The answer, I believe, lies in the combination of the multiple elements in your mind as one element, and that element "is" the singular focus or question.
Peanut butter and jam is my favourite sandwich. (I think of peanut butter and jam as a mixture here)
Peanut butter and jam are essential elements in my favourite sandwich. (Here, I think of them as separate entities)