In mixing bowl beat butter, sugar and 1 egg until thick. Add sour cream. Add flour, baking power and baking soda. Beat until smooth. Butter and flour a 9" cake pan with removable rim, and spread mixture in the pan. Bake at 350 deg. until cake is pale golden and springs back (20 to 25 minutes).
Meanwhile, in a bowl beat cream cheese, powered sugar and remaining egg till smooth. Spread over cake and arrange blackberries and blueberries on top. Bake until topping is set and lightly brown but jiggles when pan is gently shaken (20 to 30 minutes). Cool on rack. Loose edges with knife and push out of pan. Serve or refrigerate up to 1 day.