Add butter, peanut butter and buttermilk to a medium saucepan over medium heat and bring to a boil. Remove from the heat.
Whisk in powdered sugar and vanilla to a spreading consistency, or use an electric hand or stand mixer, if preferred. Pour peanut butter icing over over warm cake and spread to cover edges using a rubber scraper or offset spatula. Allow to rest for a few minutes before slicing and serving.