Scoop the batter out onto two half sheet pans lined with parchment paper. Scoop using two tablespoons, making rounds. Bake for 12-14 minutes, or until the cakes are set. Use your finger, gently touching the top of the cakes to be sure. Let cool completely. Spread peanut butter frosting on the flat side of the cooled cake and sandwich together with another cake.
In a large mixing bowl, use the beater attachment to cream together shortening, cream cheese, peanut butter, and powdered sugar. Add salt and whole milk until whipped and creamy. Adjust the milk as needed.