Do a forum search for forums. This was a topic last week or the week before. Someone posted a link for filling recipes. They had every one that you can think of. Bet they had Peanut Butter.
If you just want to use a simple peanut butter filling I made a peanut butter buttercream once to ice a cake with. You could use it for filling. I just use my regular buttercream recipe (at the time it was Wilton's class buttercream with all shortening) and subbed some peanut butter for some of the shortening. I also added peanut butter flavoring oil. It turned out really good!
HTH,
Heather
The recipe I have is:
Peanut Butter cup filling
1 cup smooth peanut butter
1 cup powdered sugar
1 Tablespoon soft butter
1 teaspoon Vanilla
Mix all ingredients on low speed till combined.
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