Kakeladi truly made a great suggestions on how to do it, but make sure that your buttercream is at room temp as well before doing so. I myself will definitely try what she said crumb coating the cake PB first...
But I do have this recipe as well that I've used a few times now and I wanted to share it with you guys as well...
Chocolate peanut butter frosting
3 cups powdered sugar
1/3 cup creamy peanut butter
2 teaspoons vanilla
1/4 cup milk (or more if needed)
3 ounces unsweetened baking chocolate, melted and cooled
1 Mix powdered sugar and peanut butter in medium bowl.
2 Stir in vanilla, milk and chocolate.
3 Beat until smooth and spreadable (start with 1/4 cup of milk and add a bit more if you need to).
Hope that helps somehow as well