Can you help me understand something? How does the Rococoa Cake cake look perfectly round in the photo when the recipe says to assemble it from vertical slices of a round cake laid horizontally? And wouldn’t some of the vertical cake slices be smaller than others?
Nigella's Rococoa Cake (from FEAST) is made up of thin slices of chocolate cake that are layered with a zabaglione-mascarpone cream. The cake is cut into thin slices and then placed in a round springform tin. The slices will be different sizes but you will need to cut the larger slices to fit and then use smaller or odd-shaped pieces to patch any gaps. As Nigella mentions in the method, don't be worried if pieces of the cake break up when you slice the cake. It is a little like making a lasagne, where you will cut or break the sheets of pasta to fit the dish.
As the cake is moistened with an espresso syrup the slices should be fairly easy to press together into the tin and the mascarpone mixture will keep the finished cake together once the cake has been chilled overnight.