Heat your oven to 180ºc/160ºc fan/350ºf and line a cupcake/muffin tin with 12 cases or put 12 baking cups onto a flat tray
Add an Oreo to the bottom of each cupcake case
Beat the unsalted butter with the light brown soft sugar until fluffy.
Add the eggs, self-raising flour and cocoa powder with the butter/sugar mix until well combined.
Split the mixture between the cupcake cases evenly.
Bake in the oven for 18-22 minutes until baked through. Leave to cool fully on a cooling rack
For the Buttercream!
Beat the unsalted butter for a couple of minutes to really loosen it.
Gradually add the icing sugar on a slow speed until it is combined then speed the mixer up and beat for 3-4 minutes until fluffy and smooth.
Make sure the Oreos are in a really fine crumb and add to the buttercream.
Beat well until smooth - add in one tablespoon of boiling water at a time mixing well, until you reach your desired consistency.
For the Decoration!
Using a 1M piping tip, or whatever piping tip you prefer (with no fine details because of the Oreo crumbs), pipe the buttercream onto the cupcakes!
Sprinkle on some oreo crumbs, and add on a whole Oreo!
Notes
The biscuits does thicken the buttercream quite a bit so I always have to add in some boiling water or whole milk to loosen it, but that is no bad thing. Always add any extra liquid a little at a time so as not to over do it.
When piping these cupcakes you have to make sure to use the correct piping tip - my normal 2d Closed star won't work because of the bits of biscuit! I use a medium or large 1M piping tip instead and it works wonders.
These cupcakes will last for 3-4+ days at room temperature, and they can freeze for 3+ months.