In large bowl, place contents of peanut butter filling pouch. Add softened butter and powdered sugar. Beat with electric mixer on medium speed until crumbly. Add milk, and beat again until smooth. Add 1/2 cup whipped cream frosting, and beat until light and fluffy. Frost cooled chocolate cake with thin layer of frosting, and refrigerate until set. Frost cake with second layer, making surface as smooth as possible.