whisk the butter and sugar together with electric beaters, until smooth and creamy, add the eggs and beat until combined, add the flour and mix. Finally add three quarters of the chocolate and all the blueberries, mix gently until they are dispersed.
Grease a cake tin and spoon the mixture in and bake at 180 degrees Celsius for approx half an hour or until the cake is golden on top and a knife comes out clean if inserted into the middle of the cake.
Meanwhile make the butter icing. Whisk the butter until nice and pale, then add the icing sugar, making sure to beat out any lumps. Carefully add the milk, a little at a time as this can curdle the mixture. Add the vanilla extract and mix again.
Once the cake has been take out of the oven, removed from its tin and cooled completely. Half the cake horizontally and spread half the icing on the bottom part of the cake and half on the top. Finally scatter the remaining white chocolate on the top.