Preheat oven to 200ºC/ 180ºFan/ Gas mark 6.
Line two cupcake trays with 18 cupcake cases.
Add a whole Oreo to the bottom of each cupcake case.
In a large bowl, sift 40g of cocoa powder and add 4 tablespoons of boiling water.
Mix the cocoa powder and boiling water to form a smooth paste.
Into the bowl add 3 large eggs, 175g softened butter, 165g golden caster sugar, 115g self-raising flour and 1 teaspoon of baking powder.
Using an electric whisk or KitchenAid with the paddle attachment (speed 6) beat the ingredients until completely smooth about 2-4 minutes. You can also mix by hand but it will take longer.
Using an ice cream scoop or spoon, divide the cupcake batter evenly between the cupcake cases. Only fill 3/4 full.
Bake in the centre of the oven for 12-15 minutes (see baking tips) Once baked remove from the oven and allow to cool completely.
Use an electric whisk or KitchenAid with the paddle attachment (speed 6) to make the buttercream. You can make the buttercream by hand but it is quite hard work and will take longer.
Beat 300g of softened butter until it's smooth then add 600g of icing sugar a spoonful at a time. Continue to beat for 3-4 minutes (see baking tips)
Blitz 6 Oreos in a food processor or crush by hand using a freezer bag and rolling pin.
Add the Oreo crumbs to the buttercream icing and mix thoroughly.
Add the buttercream to a piping bag fitted with an open star tip. I use Wilton's 1M tip (affiliate link)Once cool, pipe the Oreo buttercream in a swirl on top of each cupcake.
Decorate with a whole Oreo cookie poked into the top of the icing.