Heat the oven to 190°C (375°F) and butter and baseline 2 x 20cm sponge tins with baking parchment.
Put all the cake ingredients in the bowl of a food processor and cream together. Puour the mixture into the cake tin, smoothing the surface.
Bake for 25 minutes or until a skewer inserted into the centre comes out clean. Take out and let cool a bit. Run a knife around the collar of the cakes and leave to cool completely. When cooled, cut the bottom lengthways so that you have two equally thick cake layers.
To make the filling:
Break the dark and milk chocolate into a large, heatproof bowl and set over a pan with a little simmering water in it, stirring until melted. Remove the bowl from the pan of water and leave to cool for 5 minutes.
Once cool, whisk the chocolate, adding the cream as you go until thick and glossy. Spread the filling over the bottom sponge and sandwich together with the top sponge.
To make the icing
Gently melt the chocolate and butter in a bowl set over a pan with a little simmering water in it, stirring until smooth.
At the same time, combine the cocoa, water and golden syrup in a small saucepan and heat almost to boiling point, stirring until smooth. Add this to the melted chocolate and blend to a smooth icing.
Working quickly, smooth over the top of the cake using enough so that it drips down the side.The glaze needs to be prepared and used straightaway. If it starts to appear oily, simply whisk in 1tsp of water until it returns to being glossy.