How to salvage hardened peanut butter from jar bottom
As long as the peanut paste at the bottom of the jar hasn't gone rancid, I'm pretty sure the bottom of the jar gets hardened and dry because the peanut solids have sunk to the bottom while the peanut oil has floated to the top, and as you use the peanut butter, a deficit of peanut oil develops at the bottom of the jar. I usually try to keep re-mixing the jar as I use it, so I haven't really tried this, but I'll bet if you stir in a little peanut oil, that dry peanut-plaster would soften up and become like its old self again.