I grabbed the internet and had a search for peanut butter fudge. Most of the recipes seemed to use marshmallow or the microwave, neither of which were solutions I was interested in.
Time for Harold McGee. As suspected he had a whole chapter about sweets and in it was a nice bit of theory about fudges & fondants. This made me think a straight forward substitution of peanut butter for butter would work, a little more thinking led me to the conclusion a mixture of the 2 would be better, so half & half it was.
Next time it'll be 75% butter 25% peanut butter. There was one big problem, peanut butter doesn't lubricate as well as actual butter & one small problem, the end product is very rich indeed.Still people are enjoying it so not a complete disaster.