Preheat oven to 160ºC. Grease and flour a 25cm ring tin.
Place egg whites and cream of tartar into TM bowl and whip 6 min/Butterfly/50ºC/speed 3/MC off until peaks form. Remove Butterfly, set aside egg mix.
Place chocolate and nuts into TM bowl and mill 10 sec/speed 8.
Add butter, sugar and vanilla into TM bowl and mix 1 min/speed 4, scraping down sides of bowl if necessary.
Add egg yolks one at a time through lid onto rotating blades and incorporate 1 min/speed 4.
Add flour and sour cream and mix 20 sec/speed 6.
Add the egg whites and mix 3 sec/speed 6. Finish incorporating egg whites with the spatula.
Pour into prepared tin.
Bake for 45-50 minutes then leave to cool completely in tin. Tip out onto serving plate, top with berries and dust with icing sugar or cocoa.