My mom used to spread peanut butter on the bottom slice and dairy butter on the top slice for the jelly proofing.

this is the correct answer, especially if you're making lunch for later... or using that cheap-ass walmart bread you can basically see through.

Jelly proofing sounds like something a novice adventurer does in an RPG to defend against slime-type monsters.

But then I get peanut butter on the jelly knife

This is the only correct answer. If you make the PB a little thicker around the edges, it will create a sealed PB pocket to contain the jelly for transport.