Roughly chop your Oreos (12 Oreos). I cut mine into quarters, but I don't worry if they break up a bit more. It's nice to have some larger and some smaller chunks.
Put the rest of your ingredients into a large bowl and beat together (165g self-raising flour, 1½ level tsp baking powder, 165g caster sugar, 165g margarine or butter, 3 medium eggs, ¼ tsp vanilla extract). I beat mine either by hand or with an electric mixer on a low speed, until the ingredients are combined and there are no lumps in the margarine/butter.
Fold in the chopped Oreos.
Divide the cake mixture between the 12 cupcake cases. I like to use a cupcake scoop as it's the easiest way to evenly split the mixture.
Bake your cupcakes for c. 25 minutes until a skewer inserted into the centre come out clean.
Once baked, remove your cupcakes from the baking tray and place them on a wire rack to cool completely before decorating.
Make the buttercream
Finely grind your Oreos (14 biscuits) - This is best done in a food processor as it allows you to easily grind them very finely, but you could also use a pestle and mortar, or pop them into a plastic bag and bash them with a rolling pin.
Beat your butter (80g) until it is soft and then gradually add the icing sugar (125g), beating in each addition before adding more. I prefer to use an electric mixer on a low speed as the mixture can be quite stiff. However, this can also be done by hand.
Add the cushed Oreosand beat them into the butter and sugar. You'll find that the addition of the Oreos makes your buttercream quite stiff. Beat in a little milk (no more than 1 tsp at a time) until your buttercream is a softer, more pipeable consistency. Make sure each addition is fully combined before adding more.
Pipe the buttercream onto your cupcakes. I use a piping bag fitted with a JEM1B piping nozzle. If you don't have a piping bag then you can simply scoop a spoonful onto each one.
Top each cupcake with an Oreo.
The quantities shown for the buttercream allow you to decorate the cupcakes as shown in the images. If you decide to opt for a different style of decoration the quantity you need may vary.