Peanut butter filled Whoopie Pies

Ingredients for Cakes:
2 cups flour
1/2 cup dark cocoa powder
1 1/4 teaspoons baking soda
3/4 teaspoon salt
1/2 cup butter, softened
1 cup brown sugar
1 teaspoon vanilla
1/2 teaspoon instant espresso powder (instant coffee also works)
1 xl/jumbo egg
1 cup buttermilk
1/2 cup mini chocolate chips* optional

Ingredients for Peanut Butter Filling:
1 cup smooth peanut butter
1/2 cup butter, softened
1 cup 10x sugar

Directions for Cakes:
Preheat oven to 350 degrees.
Line baking pans with parchment paper or silicone baking mats.
Sift flour, cocoa powder, baking soda & salt.
Beat together the butter, brown sugar, vanilla & espresso powder with an electric mixer until light & fluffy (about 2 minutes).
Add egg & beat.
On low speed, beat in buttermilk... it will look curdled.
Gradually add the dry ingredients, beat until smooth.
Fold in chocolate chips.
Use a scoop (Pampered Chef or ice cream) and portion out cakes (leaving plenty of room for them to expand.
Bake 10 -15 minutes or until the tops spring back when touched.
Let cool on pan 3-5 minutes before moving them to a wire rack.
Cool completely.

Directions for Filling:
Beat together, peanut butter, butter, 10x sugar & salt on low speed until smooth.
Continue beating on med/high speed until filling is light & fluffy.
Dollop onto the bottom of a cake & top with another cake, repeat until all cakes are sandwiched together.