Preheat your oven to 180C/160 Fan/350F and line a cupcake/muffin tin with 12 cases
Beat the butter with the light brown sugar until fluffy
Combine the eggs, self-raising flour and cocoa powder with the butter/sugar mix until well combined
Using an ice-cream scoop (to make it the same amount per cupcake) dollop into the cupcake cases – once dolloped, press two sandwiches peanut butter cup miniatures together and lightly push into the cupcakes.
Bake in the oven for 18-22 minutes or until they are springy to touch and don't make a bubbling sound – leave to cool.
For the Decoration
Beat the unsalted butter with an electric mixer for a couple of minutes to loosen it.
Add the peanut butter and half of the icing sugar on a slow speed until it is combined then speed the mixer up and mix for 1-2 minutes until thoroughly combined and smooth.
Add the rest of the icing sugar and beat again. If its really thick, add in 1tbsp of boiling water at a time until smooth and pipeable.
Using a medium 2D closed star piping tip, pipe the buttercream onto the cupcakes – add on half a peanut butter cup, and some Reese's Pieces, and enjoy!
Notes
You don't have to add the Reese's Miniatures if you don't want to, but it adds a nice peanutty flavour!
I used supermarket own peanut butter, but its smooth so it pipes well.