Put the caster sugar and water in a saucepan over a medium heat for about 3 - 5 minutes, until the sugar has dissolved, then turn off the heat. Break the chocolate into pieces then add them, and the butter, to the pan and stir until both have melted. It will be a liquid mixture, a little oily looking from the butter.
Pour into a mixing bowl and leave to cool for between 30 minutes and an hour, until it is roughly room temperature.
Preheat the oven to 160°C fan (180°C for a non fan oven).
Grease an 18 cm round (or 15-16 cm square) cake tin, line the base and sides with baking paper, with the paper wrapped around the sides 2-3 cm higher than the top of the tin.
Once the chocolate mixture has cooled, sift in the plain flour, the self raising flour and the cocoa powder. Whisk on a low speed, then add the eggs and whisk again until smooth.
Pour the mixture into the prepared tin. It will almost completely fill the tin, but this is fine as it does not rise much.
Bake for 1 hour 20 minutes to 1 hour 30 minutes, until it is risen ever so slightly, and firm on top, and a skewer inserted into the centre of the cake comes out clean.
Leave to cool in the tin for at least an hour before turning out onto a wire rack.
When completely cool, simply dust with icing sugar, or it can be decorated, the same day, or up to two days later if you wrap it well in foil, with any type of icing that you wish (easy to cut into 2 or 3 layers if you want to sandwich together). If using rolled fondant icing, I suggest a thin coating ('crumb coat') of white chocolate ganache to provide a smooth surface and to help it to stick.
White chocolate ganache for a 'crumb coat' for the above cake (double if you want to fill the centre with a generous layer of ganache):
Put 75 g good quality white chocolate (drops, chips or a chopped up bar) in a heat proof bowl. Heat 50 g double or whipping cream to the point where bubbles are just starting to form around the edges, but before it boils, and pour immediately over the chocolate. Stir well until the chocolate has melted. Place in the fridge for at least half an hour, then whisk with an electric hand whisk until it has the consistency of thick buttercream. Spread a thin layer all over the top and sides of the cake, then chill in the fridge for at least half an hour, until tacky to touch. It is then ready to cover in fondant.