In the bowl of a hand or stand mixer fitted with a paddle attachment, cream the sugar and butter together until light and fluffy, 3-4 minutes. Add in the peanut butter and cream for another 1-2 minutes. Add in the vanilla extract and a pinch of salt. Add in half of the powdered sugar and whip until combined. Add in the rest of the powdered sugar and whip again until fully incorporated, scraping down the sides as needed. While mixing on low, add in the cream and mix until combined. If the frosting is too thick, add in one tablespoon of cream at a time until desired consistency if reached.
Transfer to a piping bag fitted with a tip of choice. You can also spread the frosting between the cookies with a knife. If you plan to make the frosting ahead of time keep chilled until an hour before you plan to use it, allow to come to room temperature before piping.
For Meli's Monster Cookies:
In a medium mixing bowl, cream together the softened butter and peanut butter. Add in the egg and mix until combined. Add in the dry mix and stir until mixed thoroughly.
Preheat oven to 350 degrees F, line a cookie sheet with parchment paper or a silpat mat.
Using a measuring spoon or cookie scoop, scoop even sized balls of cookie dough - roughly 1 1/2 tablespoons per cookie. Arrange evenly on a lined cookie sheet, leaving 2 inches between each cookie. Bake for 7-9 minutes until golden brown, cookies will still be slightly soft in the middle. Allow to cool for a few minutes on the cookie sheet before transferring to a cooling rack.
Once the cookies have cooled completely pipe the peanut butter frosting and sandwich together. They are best eaten immediately but will keep in an airtight container for 3-4 days!