Place the butter and peanut butter into a medium bowl, and beat with an electric mixer.
Gradually mix in the sugar, and when it starts to get thick, incorporate milk one tablespoon at a time until all of the sugar is mixed in and the frosting is thick and spreadable.
Beat for at least 3 minutes for it to get fluffy.
Spoon a few tablespoons full of frosting on top of one cookie.
Take the other cookie and press down so icing comes out to sides.
Take a knife and run it around the edge if you want a smooth edge.
Recipe by Healthy By Heather Brown at https://www.mylifewellloved.com/pumpkin-peanut-butter-cookie-sandwiches/