Cream together the butter and sweetener with an electric mixer. Add in the rest of the ingredients and mix until smooth. Using a large cookie scoop drop 16 blobs of dough onto parchment lined baking sheets. Bake for 25-27 minutes until the whoopie pies feel firm to the touch and spring back when lightly pressed. Cool completely.
While they are cooling you can make the filling. Cream together the butter, cream cheese, and sweetener. Mix in the peanut butter powder and vanilla until smooth.
To fill the whoopie pies flip half of them upside down. Spoon or pipe the frosting evenly between them. Top with a second cookie. Store in the fridge.