Pre-heat your oven to 160C Fan/180C/350F/Gas Mark 4, and line a cupcake tin with cupcake cases and put an Oreo biscuit in the bottom of each cupcake case
To make the sponge, mix together the butter and sugar, ideally using an electric mixer for 3-5 minutes
Add the eggs and vanilla extract and mix in
Then gently mix in the self raising flour
Divide the mixture between the 12 cupcake cases
Bake them for 20-25 minutes or until they are golden brown and a thin skewer inserted in the centre of the cupcakes comes out clean. Leave to cool completely
Make the buttercream, mix the butter on it's own for a few minutes, then add the icing sugar, vanilla and milk. For the best results use an electric mixer. If the buttercream is too stiff, you can add a little more milk
Crush the Oreos for the buttercream into crumbs using either a food processor, or in a bowl with the end of a rolling pin, then stir them into the buttercream
Pipe the buttercream onto each cupcake using a piping bag fitted with a nozzle. Or spread on with a spoon. Then add an Oreo biscuit on top. The biscuits will go soft after being in contact with the buttercream for a while, so add the biscuit on top as close to serving as you can
Store in an airtight container in a cool place, eat leftovers within 3 days