Heat the oven to 180°C/160°C fan/350°F/Gas 4.
For the sponges, grease and base line 2 x 20cm sandwich tins with butter and baking parchment. Combine the chocolate, butter and water in a heat-proof bowl and set over a pan of barely simmering water. Stir until melted and then whisk to combine. Remove the bowl from the heat and leave to cool slightly.
In a large mixing bowl use a balloon whisk to mix the eggs, sour cream, oil and vanilla until combined. Add both sugars and whisk until smooth. Add the melted chocolate mixture and whisk again until smooth.
Sift the flour, cocoa, bicarbonate of soda and salt into the bowl and whisk until thoroughly combined. Divide the mixture evenly between the prepared tins and level with a palette knife. Bake for about 35-40 minutes until a wooden skewer inserted into the centre of the sponges comes out clean.
Leave the sponges to cool in the tins for 10 minutes and then carefully remove from the tins and leave to cool completely on a wire rack.
For the ganache measure the chocolate and butter into a heatproof bowl and set aside. Pour the cream into a medium saucepan and slowly bring just to the boil. Remove from the heat and pour over the chocolate and butter. Leave the hot cream to melt the chocolate without stirring for 1 minute and then stir until smooth and glossy. Once smooth, leave it to cool and thicken before using.
Place one of the sponges on a cake stand and spread approximately 6 tablespoons of the ganache over the top with a palette knife or spoon. Top with the second sponge, turned upside down so the flat side is the top of the cake. Gently press the two cakes together. Cover the top and sides of the cake with the remaining ganache and roughly smooth the sides. Arrange the raspberries on top.