Pre-heat oven: 180°C / 160°C (fan) / Gas Mark 4 / 350°F
Add the butter, white chocolate, sugar and milk to a medium saucepan, and heat on a medium heat. Stir until the mixture is completely melted and smooth, then turn off the heat and set this to one side.
Beat the eggs and vanilla in a large mixing bowl until fluffy, then slowly pour in the cooled chocolate mixture, beating until combined.
Sieve in the flour and baking powder, folding in gently until only just combined.
Split the cake mix evenly between two greased and lined 8inch cake tins and bake for 25-30mins until lightly golden and skewer comes out clean from the centre.
Allow the cake to cool for 20mins before removing from the tins and transferring to a wire rack to finish cooling. While the cakes cool, make the frosting.
Blend the raspberries, lemon juice and water until you get a smooth puree.
Sieve the pureed mix into a saucepan, discarding the pulp, and heat over a low-medium heat. Simmer for roughly 15-20mins until it has reduced to a thick liquid. Allow to cool to room temperature.
While the raspberry reduction cools, beat the butter until pale and fluffy.
Add half of the icing sugar to the butter and beat until combined, before adding the remaining icing sugar and beating again until smooth.
Add the raspberry reduction to the frosting 1 tablespoon at a time, beating until smooth, until you have the desired colour and flavour (I usually use 2-3 tbsp).
Once the cake sponges have cooled, turn one sponge upside-down and spread roughly ⅓ of the frosting on top.
Place the second sponge on top and cover the cake in the rest of the frosting. Decorate with fresh raspberries and enjoy!
* You can swap the two 8-inch cake tins for two 6-inch cake tins and bake for an extra 5-10mins. To swap to two 9-inch cake tins, increase the recipe by 1 and bake for an extra 5-10mins.** To swap the plain flour for self-raising flour, use the same amount of flour and exclude the baking powder.