Peanut Butter Cup Whoopie Pies with Chocolate & Peanut Butter Glaze

Ingredients
For the cookies:
3 1/2 cups all-purpose flour
1/4 teaspoon salt
1 1/4 teaspoons baking powder
1 1/4 teaspoons baking soda
3/4 cup dark unsweetened cocoa powder
2 teaspoons instant espresso powder (NOT instant coffee)
1/2 cup hot coffee
2 cups firmly packed brown sugar
3/4 cup canola oil
1 large egg
1 teaspoon pure vanilla extract
1/2 cup buttermilk, shaken
For the filling:
3/4 cup butter
3/4 cup creamy peanut butter
3/4 cup powdered sugar
1/2 teaspoon salt
For the chocolate & peanut butter glaze:
2 tablespoons unsalted butter
2 tablespoons creamy peanut butter
3 tablespoons light corn syrup
1 teaspoon vanilla extract
6 ounces semisweet chocolate chips
1/4 cup powdered sugar
20 miniature REESE'S peanut butter cups, coarsely chopped
Directions
To prepare the cookies, preheat oven to 350 degrees Fahrenheit. Line two baking sheets with parchment paper.  In a large bowl, sift the flour, salt, baking powder, and baking soda.  Whisk to combine. Set aside.  In another large bowl, sift together the cocoa powder and espresso powder. Whisk to combine.  Add the hot coffee and 1/2 cup hot water and whisk until both powders are completely dissolved.  In a medium bowl, stir together the brown sugar and oil.  Add this to the cocoa mixture and whisk until combined.  Add the egg, vanilla, and buttermilk and whisk until smooth.  Use a rubber spatula to gently fold the dry ingredients into the wet ingredients. Make sure to scrap down the bowl as you fold.  Use a small ice cream scoop with a release mechanism to drop heaping tablespoons of the dough onto the prepared pan, about 1 inch apart (if using baking sheets).  Bake for about 10-15 minutes until the cookies are just starting to crack on top. Let the cookies cool completely on the pan while you prepare the filling. If you are using a whoopie pie pan, let the cookies cool for 5 minutes, then transfer to a wire rack to finish cooling (otherwise they will get stuck inside the pan).
For the filling, combine the butter and peanut butter in the bowl of a stand up mixer. Mix on low until creamy.  Sift the sugar into the bowl and add the salt. Mix on low to incorporate.  Increase speed to medium and beat for about 4 minutes until the frosting is light and fluffy. 
 
To prepare the glaze, combine butter, peanut butter, corn syrup, vanilla, and 2 tablespoons water in a microwave-safe bowl.  Heat in a microwave oven on High 1 to 1 1/2 minutes, until butter is melted and mixture is smooth when stirred.  Add chocolate chips and stir until completely melted and smooth.  Sift in powdered sugar and stir until blended.  Refrigerate 10 minutes, or until glaze is of spreading consistency.
To assemble, transfer the peanut filling and glaze into separate piping bags. I used a number 12 tip for the filling and a number 3 tip for the glaze.  Here is a tip to easily fill your bags. With the tips in place, fold top cuff of the bag over the side of the glass. Fill your bag half way full, twist securely and hold the end of the bag in place with a rubber band. 
 
*Instead of using piping bags you can alternatively use an offset spatula to ice the cookies with the filling. Transfer the glaze to a small zip lock bag, cut off a tiny portion of the corner and pipe the glaze over the filling.
Place one cookie flat side up. Place a generous dollop of filling on the cookie (not out mountain or it will squish out the sides when you bite into it). Add a drizzle of the chocolate peanut butter glaze.  Top with another cookie and roll edges in the chopped peanut butter cups.
 
Wrap whoopie pies in plastic wrap and store in an air-tight container. These also freeze beautifully.
 
Cookie recipe courtesy of Baked Explorations
Filling recipe courtesy of Whoopie Pies by Amy Treadwell
Glaze recipe courtesy of Recipe Source