Preheat your oven to 160C/fan 140C/gas 3. Grease and line 2x20cm sandwich tins.Mix the flour, sugar and cocoa together. Melt the chocolate and butter and 200 ml water together over a low heat, then beat this along with the eggs into the dry ingredients.Pour into the two sandwich tins, and bake for approximately 40 mins, until well risen, and a skewer inserted in the middle comes out clean. Transfer to wire racks and allow to cool.

While the cake is cooling, make the buttercream. Beat the butter until creamy, then add 300g of the icing sugar and vanilla. Mix well. An electric mixer is really useful here! You may find the mixture a bit dry - if so, add a tablespoon of milk at a time. If the mixture is too wet, add a little more icing sugar. Blitz all but one of the cookies in a food processor. Mix half of the cookie crumbs into the buttercream, and reserve the other half.

Spread the butter icing between the two cakes, and over the top of them when sandwiched together.Sprinkle the remaining cookie crumbs over the top. You could add mini oreos to decorate of you like!