Step 4: Make the Filling and Assemble
Cream together butter, peanut butter, cream cheese, powdered sugar and vanilla with a mixer set to medium speed for a full 5 minutes, scraping the bowl periodically with a rubber spatula.
Transfer mixture to a ziplock bag with the corner clipped off, or a pastry bag with a plain tip. Or just use a spatula and spread between the cookies, as I did (I wasn't using any fancy shapes).
Pipe or spread filling onto the flat side of half of the cookies. Top with the remaining cookies, gently sandwiching the filling in between.
Since I forgot to make the designs on the tops of the cookies before baking, I thinned the remaining frosting with water and piped it on afterwards!