Break the plain chocolate into pieces and melt it in a bain-marie with the butter, without letting it heat too much. Separate the egg whites from the yolks, then beat the latter with the sugar until you obtain a smooth, creamy mixture. Add the sifted flour and, a little at a time, the melted chocolate and butter, stirring constantly. Beat the egg whites until stiff (you can use an electric mixer if you like) and then add them to the rest, stirring with a wooden spoon from bottom to top, a couple of spoonfuls at a time. When the egg whites are mixed in well, pour the mixture into the buttered and floured cake tin and cook in a preheated oven, at 160 °C, for approximately 35 minutes. Take the cake out of the oven and let it cool down. Using a large, serrated knife (such as a breadknife) cut it into two disks. Carefully, so as not to break it, place the upper part on a plate and, with the help of a spatula, spread the Viallella (after you’ve stirred it well) evenly on top of the other half. Position the “lid” that you removed before on top of the chocolate spread and then prepare the ganache icing. Put the cream in a small saucepan and bring to the boil. As soon as it boils, turn off the heat, add the chocolate broken into small pieces and mix well with a whisk. Now place the cake on a sheet of greaseproof paper (so that it’s easier to remove the excess icing) and pour the ganache over it. To distribute it over the whole surface of the cake, including the edges, the chocolate must be “poured” very slowly. Leave to cool, so that the ganache icing solidifies, and you can detach the cake from the paper and transfer it onto the serving dish – it’s beautiful to look at and delicious to eat!