Preheat the oven to 170C/160 fan. Line a muffin tray with paper muffin cases.
Put the flour, sugar, cocoa powder, baking powder, salt and butter in a large bowl and mix until you get a sandy consistency and everything is combined.
Whisk the milk, eggs, and vanilla extract together in a separate jug, then slowly add half into the flour mixture, and beat well to get rid of any lumps. Gradually add the rest of the milk mixture whilst mixing until smooth.
Add the mixture to the paper cases, making sure they are equally full (about two-thirds). Bake the cakes in the oven for 20-25 minutes.
Whilst the cakes are cooking, smash up 6 Oreos in a bag and set aside. Whisk the butter, cream cheese and icing sugar until light and fluffy. Then, gently fold in the Oreo Crumbs.
When the cakes have cooled, pipe the icing onto the cakes and use any left over Oreos to decorate.