Preheat oven to 350°F. Prepare Mini Whoopie Pie Pan with vegetable pan spray.
In large bowl, stir together flour, baking soda, baking powder and salt.
In large bowl, beat butter and sugar with electric mixer until light and fluffy. Add peanut butter, egg and vanilla extract; beat until well combined. Alternately add flour mixture and buttermilk in three additions, beating until just combined. Spoon one tablespoon batter into each pan cavity.
Bake 9-11 minutes or until tops of cakes spring back when touched. Cool in pan 3 minutes. Cool completely on cooling grid.
For buttercream, combine shortening and butter in large bowl. Beat with electric mixer until light and fluffy. Add melted chocolate and vanilla; beat well. Gradually add sugar, one cup at a time, on medium speed; scrape bowl often. Add milk and beat on high speed until light and fluffy.
To assemble: Place one cake in bottom of treat pop. Pipe a swirl of buttercream from back edge following the curve of the container, then filling in the center. Pipe a second swirl of buttercream in the center leaving open at the container edge . Fill in open space with chopped peanuts. Continue layering until treat pop is full. Top with Tip 21 swirl of chocolate buttercream; top with additional chopped peanuts.