Interesting. I imagine the peanut butter helps hold it together as well as adding a peanut flavor?
I just did peanut butter on the bun actually - I picked up the tip from a local restaurant and haven't looked back. I figure they've won Best Burger in the City for the last 5 years, so they must be doing something right - and perhaps it's the PB.
I would imagine peanut butter would probably work in nicely with the beef prior to cooking though. Perhaps I'll try that next time. We didn't throw anything extra into the ground beef this time - just pure cow.
If I did add some extra PB to the mix beforehand, I'd definitely either go for the organic **** (which is king) or the Unsweetened/Unsalted Kraft (slightly worse but still tasty). The regular ol' Kraft would be a little overwhelm the ground beef a little bit methinks.