Preheat your oven to 350°F (177°C) and line a 12-cup muffin tin with cupcake liners.

In a small bowl, whisk together the cocoa powder, flour, baking powder, baking soda, and salt until evenly combined.

In a larger bowl, cream the butter and sugar using an electric mixer until the mixture is light and fluffy. Add the egg and vanilla extract, and mix briefly until just combined.

Stir in the sour cream and the cooled, melted chocolate using a spatula until smooth.

Gradually add the dry ingredients to the wet mixture in two parts. Rather than dumping all the flour mixture in, lightly sprinkle spoonfuls over the wet ingredients, as if sifting it. Fold gently until no dry streaks remain, scraping the sides and bottom of the bowl to ensure everything is incorporated. Repeat with the remaining dry mixture. Set the chocolate batter aside.

In a medium bowl, mix together the flour, baking soda, baking powder, and salt.

In another bowl, combine the peanut butter, vegetable oil, and brown sugar. Beat together using an electric mixer until smooth and creamy.

Add the egg and vanilla extract, then beat again until well combined.

Add the dry ingredients and buttermilk in alternating additions, mixing by hand. Stir just until everything is fully incorporated and the batter is smooth.

Spoon the chocolate and peanut butter batters alternately into the prepared cupcake liners, filling each one nearly to the top.

Place the muffin tin in the center of the preheated oven and bake for 15 to 17 minutes, or until a toothpick inserted into the center of a cupcake comes out clean.

Let the cupcakes rest in the pan for about 3 minutes, then remove them and place them upside down on a wire cooling rack. This cooling method helps create domed tops.

Allow the cupcakes to cool completely before adding any frosting.

In a mixing bowl, beat together the butter and vegetable shortening using an electric mixer until fluffy and light.

Add the cocoa powder and mix gently with a spatula until mostly incorporated.

Gradually add the powdered sugar, 1 cup at a time. Start by mixing it in with a spatula, then switch to the electric mixer to beat until smooth. Continue adding powdered sugar until the flavor and consistency meet your preference.

In a large mixing bowl, beat the butter and vegetable shortening with an electric mixer until smooth and fluffy.

Add in the peanut butter and beat again until well incorporated.

Slowly add the powdered sugar, one cup at a time. Mix each addition by hand first, then beat with the mixer until the frosting is light and creamy. Add more powdered sugar as needed to reach your desired sweetness and texture.

Prepare a piping bag fitted with a Wilton 6B tip. Carefully add the chocolate buttercream to one side of the piping bag and the peanut butter buttercream to the opposite side. Press the buttercream down into the bag evenly.

Pipe the frosting onto each cooled cupcake, starting at the outer edge and spiraling inward toward the center. Continue layering the swirl upward to build a beautiful, stacked frosting dome.