Pre-heat the oven to 180 C / Fan 160 C / Gas 4. Grease one 23cm (9 in) deep sandwich tin then line the base of the tin with baking parchment.
Pour hot water in a pan and put it on a low heat hob so that it simmers. Place a heat-proof bowl over the pan of simmering water, ensuring the pan doesn't touch the water. Add 200g of the chocolate, mixing occasionally until it has turned smooth. Take the pan off the heat and scoop the butter, sugar and salt in, then mix together until it has dissolved into the chocolate mixture.
Wait until the chocolate and butter mixture has cooled, then add half of the beaten egg yolks and whisk in with an electric mixer or by hand. Keep doing this until all of the egg yolks have been incorporated.
For the egg whites, whisk them in a large bowl on a high speed until soft peaks begin to form. Add half of the egg white to the chocolate mixture and fold in, then do this with the other half. It should turn lighter and become fluffy.
Sift in the self-raising flour and baking powder then fold it in. Once everything is mixed in, put the mixture into the prepared cake tin.
Bake for 30 to 40 minutes, or until the top of the cake looks crisp and shiny. A skewer inserted into the centre of the cake should come out clean.
Melt the buttercream in a heat-proof bowl over a hot pan, then add the remaining 25g of chocolate. Once melted and smooth, remove from the heat and allow to cool.
When the cake is cooked pull it out of the oven and straight out of its tin, then wait until cool to decorate how ever you wish with the buttercream.
Finally, sift the icing sugar onto the cake to create a beautiful look!
The cake forms a weird shape when it cooks, so don't freak out! You will be covering it all up with buttercream and it will taste wonderful!