First of all make the zabaglione filling. Using an electric hand whisk, beat the egg yolks for 1 minute in a medium bowl, then add the sugar and beat until the mixture is thick and pale yellow (about 3 minutes). Next, whisk in the flour a tablespoon at a time, mixing in very thoroughly, then gradually whisk in the Marsala.
Now tip the mixture into a medium heavy-based saucepan and place over a medium heat. Then, cook the mixture, stirring constantly, until it has thickened and is just about to boil; this will take about 5 minutes. Don't worry if it looks a bit lumpy, just tip it into a clean bowl, then whisk until smooth again. Let the custard cool, whisking it from time to time to stop a skin forming. When it is cold, cover with clingfilm and pop in the refrigerator for at least 2 hours. Pre-heat the oven to gas mark 3, 325°F (170°C).
Meanwhile, make the cake. To do this, take a large mixing bowl, place the flour and baking powder in a sieve and sift into the bowl, holding the sieve high to give them a good airing as they go down. Now all you do is simply add the other cake ingredients to the bowl and, provided the butter is really soft, just go in with the electric hand whisk and whisk everything together until you have a smooth, well-combined mixture, which will take about 1 minute. What you will now end up with is a mixture that drops off a spoon when you give it a tap on the side of the bowl. If it seems a bit stiff, add a little water and mix again.
Now spoon the mixture into the tin, level it out with the back of a spoon and place the tin on the centre shelf of the oven. The cake will take 30-35 minutes to cook, but don't open the oven door until 30 minutes have elapsed. To test whether it is cooked or not, touch the centre lightly with a finger: if it leaves no impression and the sponge springs back, it is ready. Remove it from the oven, then wait about 5 minutes before turning it out on to a wire cooling rack. Carefully peel off the base paper, which is easier if you make a fold in the paper first, then pull it gently away without trying to lift it off. Now leave the sponge to cool completely.
To assemble the torta, whip the double cream in a large bowl until stiff, then add the zabaglione custard to the bowl and whisk again until thoroughly mixed. Place the cake flat on a board, then, holding a serrated palette knife horizontally, carefully slice it into 2 thin halves. Next, reserve 2-3 heaped tablespoons of the zabaglione filling to decorate the sides of the cake and spread the rest of the filling over the bottom half, easing it gently to the edges. Place the other cake half on top and press down very gently. Before you spread the mixture on the sides of the cake, it's a good idea to brush away any loose crumbs, so they don't get mixed up in it. Now, using a small palette knife, spread the reserved filling evenly all around the sides of the cake. Finally, dust the top with the icing sugar before serving. If the cake is made and decorated ahead of time, store it, covered, in the fridge (to keep it firm), but remove it half an hour before serving.