Preheat oven to 190 degrees.
Line baking sheets with parchment paper or foil
In a bowl, sift together, flour, cocoa, baking soda, and salt.
In another bowl, beat butter, shortening and sugar with a mixer on low until just combined. Increase speed to medium and beat for about 3 minutes.
Add egg and vanilla and beat for two more minutes.
Add half of the flour mixture and half of the milk and beat on low until incorporated. Repeat with remaining flour and milk and beat until combined.
Using a tablespoon, drop batter on baking sheet two inches apart. Bake for about 10 minutes each or until pies spring back when pressed gently.
Remove from oven and cool for about five minutes before transferring them to a rack to cool completely.
Beat peanut butter and butter on low until creamy using a mixer.
Add sugar and salt and beat on low to incorporate. Increase speed to medium and beat for about four minutes until the filling is light and fluffy.
Spread filling on flat side on one cooled cake or use a pastry bag with the 1M tip to make the swirly look. Top it with a second cake and press gently.