Brooklyn blackout cake

Method

1. Preheat the oven to 180°C. Wrap foil around the outside of two 20 cm round springform cake tins to prevent any batter dripping out, then grease and line the bases with baking paper.
2. Sift the flour, cocoa, bicarbonate of soda, baking powder and salt into a large bowl. Using an electric mixer, beat the eggs and sugar for 2 minutes or until thick and pale. Add the oil and beat until well combined. In batches, alternately add the flour mixture and sour cream, beating on low speed until just combined. Add the coffee and vanilla, and stir well to combine (the batter will be quite runny).
3. Divide the batter evenly between the prepared tins, then bake for 45 minutes or until a skewer inserted into the centre of each cake comes out clean. Cool in the tins for 15 minutes, then turn out onto a wire rack to cool completely.
4. Reduce the oven temperature to 150°C.
5. Meanwhile, to make the chocolate-coffee crumbs, line a baking tray with baking paper. Sift the flour, cocoa, sugar, cornflour and salt into a bowl. Add the butter and coffee and, using an electric mixer, beat until the mixture comes together in small balls. Spread out on the prepared tray and bake for 20 minutes or until dry. Allow to cool completely, then loosely crumble and transfer to an airtight container until needed.
6. To make the Oreo buttercream, process the chocolate wafers in a food processor to fine crumbs (make sure there are no small chunks or the buttercream won’t be smooth). Add the cream and melted butter and process until moistened and well combined. Add the milk and process until smooth. In a seperate, clean bowl, using an electric mixer, beat the icing sugar and the extra 200 g of butter for 3 minutes or until light and creamy. Then, add the chocolate mixture and beat for a further 2 minutes or until smooth. Set aside at room temperature until needed.
7. To assemble the cake, using a large serrated knife, trim the tops off the cakes to make them level, then cut each cake in half horizontally. Place one base cake layer on a cake stand, then brush generously with milk. Spread over one-third of the buttercream all the way to the edges. Repeat this layering with two more cake layers, milk and buttercream. Finish with the remaining cake base, bottom side up, then brush generously with milk. Refrigerate for 30 minutes to firm slightly.
8. Meanwhile, to make the fudge frosting, place the chocolate and cocoa in a heatproof bowl. Place the remaining ingredients in a saucepan over medium heat and cook, stirring, until melted. Pour over the chocolate and cocoa, stand for 1 minute, then whisk until melted and smooth. Set aside to cool to a spreadable consistency.
9. Using a palette knife, carefully scrape and smear any buttercream around the side of the cake to smooth and achieve a ‘naked’ effect. Spread the fudge frosting over the top, then pile on the chocolate crumbs and serve.