Combine the butter and chopped dark chocolate in a medium saucepan over low heat. Cook, stirring, for 5 mins or until the chocolate melts and the mixture is smooth. Set aside for 5 mins to cool slightly. Transfer to a large bowl.
Add the sugar and eggs to the chocolate mixture in the bowl and stir to combine. Add the flour and cocoa powder and stir to combine. Stir in the sour cream until combined. Add half the white chocolate and half the frozen raspberries and gently fold to combine.
Grease a slow cooker and line the base and side with 3 layers of baking paper. Spoon the mixture into the prepared slow cooker and smooth the surface. Sprinkle with the remaining white chocolate and frozen raspberries.
Cover and cook for 3 1/2 -4 hours on low or until almost set. Uncover and cook for a further 30 mins or until a skewer inserted in the centre comes out clean. Turn the slow cooker off. Leave brownie in slow cooker for 1 hour to cool slightly. Use paper to transfer brownie to a wire rack to cool completely.
Cut brownie into pieces. Serve with shaved chocolate and torn raspberries.