Preheat the oven to 160℃ (320℉). Cut the chocolate and butter into evenly-sized cubes. Improvise a bain-marie by boiling 2cm (1 inch) water in a pan and placing a bowl over it that will sit on the rim of the pan without the bowl touching the water. Put the chocolate and the butter in the pan and simmer until everything is melted, about 5 minutes. Stir it to mix it all together. Put the bowl in the fridge.
In a large bowl, whisk the eggs until they are somewhat foamy, but not stiff. Add the sugar and vanilla (you can use vanilla powder or essence, or fresh - if using fresh you'll need to slit the pod open and scrape out the seeds). Whisk rapidly until you get a mousse-like consistency. Fold in the chocolate and butter mixture, and then sieve the flour in, stirring constantly, and finally add the salt and stir it in.
Butter the inside of your tin and pour in the batter mix. Place in the oven and bake for 1 hour. When you take it out, you can test for doneness by sticking a skewer into the middle. If it comes out dry, the cake is done. As soon as it is cool enough to handle, remove it from the tin and place on a wire rack to cool completely.
Chop your chocolate into evenly sized pieces and put them in a bowl. Bring the cream almost to the boil, and pour over the chocolate. Stir until the chocolate has all melted and place in the fridge to cool down and thicken.
Now we need to slice the sponge in half. If your sponge has a domed top, slice it off first. Turn the sponge over so that the flat bottom becomes the top. Take a long sharp knife and very carefully cut the cake horizontaly with a slow sawing motion. Set aside.
Now we need to temper some chocolate to cover the top of the cake, and to make some chocolate leaves with. Tempered chocolate is really shiny and is made by heating, cooling and re-heating chocolate. In the bain marie, heat the chocolate to 45℃ (113℉). As soon as it has reached that temperature, place the bowl of chocolate into a pan of iced water, and leave it until the temperature falls to 27℃ (81℉). Then place it back into the bain marie until the temperature comes up to 30℃ (86℉). Remove from heat.
Chocolate leaf decorations
To make chocolate leaves, you need some real leaves as moulds. I'm using bay leaves, but any small leaf will do. Wash and dry the leaves, and then paint a layer of the molten tempered chocolate onto the upper surface. Put them in the freezer until the chocolate has set and then give them all a second coat and freeze again.
Assembling the cake
Place the bottom slice of the cake on some kind of container to raise it up from the table top (if you have a turntable, that would be ideal). Spread a layer of jam on the top, and then pour or spread ganache over it (depending on whether your ganache set or not). Place the second slice of cake on top, spread with jam and ganache. Using a palette knife, spread ganache around the edge of the cake. Finely grate the white chocolate and throw it at the side of the cake until it is all coated. Pipe ganache around the rim of the cake, and then pour molten tempered chocolate on top. Peel the bay leaves away from their chocolate coating, and position artistically in the centre.