NIGELLA LAWSON'S ROCOCOA CAKE

Steps:

• If you're going to make your own cake, proceed as follows. Preheat the oven to gas mark 4/180°C Butter and line the bottom of a 23cm springform tin.
• Sift together the flour and cornflour, and add the cocoa, pushing it through a sieve. Whisk the separated yolks with half of the sugar - you can judge this by eye - until the mixture becomes pale and moussey.
• In a separate bowl, whisk the egg whites with a pinch of salt until firm, then whisk in the remaining sugar, one spoonful at a time, until you have gleaming white peaks.
• Gently fold the yolk mixture into the whites, and then add the flour, cornflour and cocoa, folding gently again until combined. Pour this moussey liquid into the tin and bake for 30 minutes. The cake will be almost silicon-springy on top. Unclip the tin and let the cake cool on a rack, right side up.
• Meanwhile, to make the syrup, bring the sugar and water to a boil in a small saucepan and let it bubble for a scant minute before taking the pan off the heat and adding the coffee or espresso powder made up with water, and the rum. Stir - just with a fork or anything - pour the hot syrup into a jug or bowl and let it cool.
• To make the filling, put the yolks, sugar and rum into a bowl that will fit over a saucepan of barely simmering water. Whisk (I use a hand-held electric mixer for this) until the mixture has thickened airily and then lift the bowl off the pan and let it sit on a cold surface while you whisk for another few minutes to help it cool down. Don't agitate yourself too much about whether it's thick enough or not: as long as it has thickened, the marscarpone and cream will give it the right texture to fill the cake with later.
• Make sure your rum-zabaglione mixture is pretty well cool before whisking in the mascarpone. Softly whip the cream in a separate bowl and beat or fold that into the filling as well.
• To assemble the rococoa cake, cut the cake you made into thin vertical slices, that's to say as if you were slicing a round loaf of bread rather than a cake. If using the bought loaf cakes, cut into slices along the length of the cakes, as thinly as you can. Don't worry about breakages: you are, after all, able to wodge everything together with the syrup in the tin. So: brush a 23cm springform tin with some of the syrup and then layer a third of the sliced chocolate cake, laid horizontally, to line the bottom of the tin. Brush again with the syrup to dampen the cake and seal the joins.
• Spread the layer of chocolate cake with half of the zabaglione using a rubber spatula and a light hand to coat evenly, and then add another layer of cake slices to cover.
• Dribble again or brush with the syrup until the cake is damp as before, and then spread over the final half of the filling.
• Cover with the final third of chocolate cake slices and drip, pour or brush over the syrup to give the cake a smoothish layer, which can be iced later; if the cake is damp, there's no call to drench it. And if the top of the cake is quite wet, don't be tempted to use all of the syrup; the bought chocolate cake is often damper and denser (and needs less syrup) than the homemade cocoa-sponge.
• Put the cake, covered with clingfilm, in the fridge overnight to set. You can ice the cake ahead of your dinner party and put the cake back in the fridge again (though see the caveat in the final paragraph below), but do let it set overnight first.
• To make the icing, put the sugar, syrup, rum and espresso powder into a small saucepan and bring to the boil. Turn off the heat and add the chopped chocolate, swirling it around so that the chocolate melts in the hot liquid. Leave for a few minutes and then whisk everything together in the pan (just using a little hand whisk) to make a smooth shiny glaze. Moving quickly, spring open your tin, taking care with the sides as the cake will be damp and delicate; you might want to run a small spatula around the inside first.
• Sit the cake on a plate or stand (don't even think of trying to loosen it from the tin's base) and pour over the icing, not worrying if it dribbles down the sides too much. You may need to ease it over the top of the cake while it is still malleable. It will set quite quickly - the fridge will have made the layer it sits on very cold - and you will ruin the finish if you try and spread the icing after your initial pouring.
• Scatter with gold sprinkles, nibbed or chopped pistachios, wafer roses, sugar flowers or any other decorations of your choice. The glaze will dull a little if you put it back in the fridge, so on the whole it's best to ice the cake and decorate it about 20 minutes before you want to eat it, or just before you sit down to dinner.