Preheat the oven to 180 C / Gas 4. Grease two deep 25cm round cake tins.
In a mixing bowl cream together the butter and sugar until light and fluffy. Slowly add the eggs one at a time and beat until creamy. Sift in the flour and add the vanilla extract and milk. Separate the mixture evenly into the prepared cake tins.
Bake in the preheated oven for 40 minutes until risen and golden brown. After 30 minutes cover with foil to prevent the top of the cake from burning. Continue to cook until a skewer inserted into the centre comes out clean. Remove from the oven and allow to cool completely on a cooling rack upside down, so that they are easier to stack for decorating.
For the frosting:
Beat the butter and icing together. Crush the Oreo® cookies into fine breadcrumbs and add to the mixture. Pour in the vanilla and milk and mix until light and fluffy.
If necessary, cut the top off the cakes so they are nice and even. Spread the frosting in the centre and then on top of the cake. Finish by sprinkling chopped up Oreo cookies and chocolate on top.