Preheat your oven to 180C (356F) or 160C (320F) for a fan forced oven. Line a cupcake tin with cupcake liners.
In the bowl of a stand mixer fitted with the paddle attachment, add the cocoa powder, plain flour, salt, bicarb soda and sugar. Turn mixer on low speed and allow it to mix for a couple minutes to help everything combine well (alternatively you may do this by sifting the ingredients together). Add the softened butter there is no butter visible and it's well incorporated into the dry.
Next, add milk and eggs in a jug and whisk until well combined.
Add wet ingredients to dry ingredients in a slow and steady stream until no dry ingredients are visible. Scrape down the bowl and mix for another 20 seconds.
Fill each paper 3/4 of the way (using an ice cream scoop to transfer the batter into the cupcake liners makes this a quick and easy process otherwise you can use two tablespoons). Sprinkle with some Reece's peanut butter chips. Bake for 20-25 min or until a toothpick inserted comes out clean. Allow them to cool completely on a wire cooling rack before frosting.
While your cupcakes are baking and cooling prepare your buttercream frosting by combining the frosting and peanut butter together.
Fit the end of your piping bag with a round tip and frost in a donut swirl as demonstrated in the video. Sprinkle with crushed peanuts and top with chopped peanut butter cups.