Rachel's Oreo cookie butterfly buns

Preheat the oven to 180ºC/350ºF/gas mark 4. Place 10 paper cases in a cupcake baking tray. Cream together the butter and caster sugar, using the paddle attachment of an electric mixer or by hand, until pale and fluffy. Add the eggs a little at a time, beating after each addition. Fold in the flour and cocoa powder. Fill each cupcake paper case half full. Don’t be tempted to overfill them as they will turn into a mess! Bake for 15 mins, or until the cakes are well risen and spring back to the touch. Remove the cakes to a wire rack and leave to cool completely. Crush 8 of the Oreos into fine pieces in a bowl, then add the butter and icing sugar, along with 2tbsp of water. Whisk for 5 - 10 mins (the longer, the better) to give the icing lots of volume. Take the cooled cakes and with a knife, slice off the top part of each cake and scoop out a small crater about halfway deep. With a teaspoon, generously fill the craters with the Oreo buttercream icing. Take the remaining 10 Oreos and cut each one in half ( I find a quick heavy cut is best and makes the cookie less likely to crumble). Push a pair of Oreo 'wings' into the icing on each cake to create the butterfly effect.