Preheat the oven to 150ºC/gas 2. Grease and line the base of a 23cm cake bottom-loose tin.
Break the chocolate into a saucepan, cube and add butter. Melt them on a low heat and mix until combined.
In a large mixing bowl, stir the flour, sugars and cocoa until well combined.
Whisk the buttermilk into the melted chocolate mixture, then add the eggs and mix them together.
Put everything into the flour-sugar mixture and whisk until you get a smooth batter.
Pour it into the cake tin and bake it for around 1 hour 15 minutes, or until cooked through. Stick a skewer in the middle of it and it should come out clean.
Let the cake cool in the tin for 10 minutes, then place it on a wire rack to cool completely.
Meanwhile, let's make the ganache:
Break the chocolate into a bowl.
Pour double cream into a small pan and heat it gently. Just before it starts bubbling, remove the cream from the heat and pour it over the chocolate (never add the chocolate to the cream because it will seize).
Mix with a spatula until completely combined. Leave the ganache to cool completely in the fridge, until it thickens. Split it, half for the filling, half for coating the cake.
Using an electric mixer or whisk, beat half of the cooled ganache for 3 to 5 minutes, until light and fluffy.
To assemble, place the cold cake on a board and cut it horizontally across the middle, using a long serrated knife.
Put one of the cake's layers onto a plate or stand, cut-side up. Put 3-4 spoons of raspberry jam and spread it evenly. On the other layer, add 3 generous dollops of white chocolate ganache. Spread it evenly.
Sandwich the sponge layers together and place the cake on a wire rack in a tin.
Pour the other half of chocolate ganache on the cake.