1. Preheat the oven to 350º F. Position oven rack in the middle of the oven.
2. Line a regular muffin tin with 12 cupcake liners and set aside.

Dark Chocolate Peanut Butter Batter:

1. In a medium mixing bowl, combine: cocoa powder, flour, baking powder, baking soda and salt, whisk and set aside.
2. In a large mixing bowl or in the bowl of an electric mixer fitted with the whisk attachment, combine peanut butter, oil, and brown sugar, beat until all combined and fluffy.
3. Add in the eggs one at a time, and vanilla extract. Beat on medium speed until combined. Stop a few times to scrape the sides and bottom of the bowl with a spatula.
4. Add the flour mixture and buttermilk, alternating between the two and mixing by hand (to avoid over mixing) until all combined.
5. Fill the cupcake liners almost completely full with batter.
6. Bake for 15-17 minutes or until a toothpick inserted in the center comes out with just a few crumbs.
7. Allow the cupcakes to cool for 3 minutes in the pan, then transfer them to a cooling rack. Place cupcakes upside down on the cooling rack to create a dome top.
8. Allow the cupcakes to cool completely before frosting them.

Peanut Butter Buttercream:

1. In a large mixing bowl or in the bowl of an electric/stand mixer fitted with the whisk attachment, combine butter and vegetable shortening, beat until fluffy.
2. Add in the peanut butter and vanilla extract, beat until fully combined on medium-high speed. Stop a few times to scrape the sides and bottom of the bowl with a spatula.
3. Begin to add the powdered sugar, 1 cup at a time, mix with the mixer speed on low after that increasing it to medium.
4. Continue adding powdered sugar until the frosting tastes sweet enough to you, I used 4 cups.

Optional:

1. Transfer 1/2 of the buttercream to another and using a mixer, mic in 1/4 cup of cocoa powder.
2. Add frosting to a piping bag fitted with a large star piping tip, if you use chocolate and peanut butter buttercream, add chocolate buttercream to one side of the bag and peanut butter buttercream to the other, it doesn't have to be perfect.
3. Pipe the buttercream onto the cupcakes, starting in the center and swirling to the outside and swirling back into the center as you progressively stack the frosting.
4. Garnish yeah cupcake with a peanut butter cup.
5. Refrigerate for at least 2 hours before serving.
6. Enjoy!