These cookies cool quickly after removed from the oven. While they are cooling down, open the container of frosting and transfer some of it into a plastic zipper freezer bag. I set my opened bag into a measuring bowl to make it easier to spoon in the frosting.
Cut just the tip of the bag and then pipe a line around the back of half the cookies before filling in completely with more frosting. You want enough frosting so some will stick out and catch more sprinkles on the side, but not too much that it’s sloppy. Top with the other cookies.