Preheat oven to 350F. Line 2 (15x10x1-inch) baking pans with foil; spray with cooking spray.
Place cake mix, eggs, 1-1/3 cups water and 1/3 cup of the sour cream in large bowl. Beat with electric mixer on low speed 2 minutes; scrape bowl. Beat on medium speed an additional 2 minutes.
Pour evenly into prepared pans.
Bake 15 to 18 minutes or until toothpick inserted in centers comes out clean. Cool completely.
Beat remaining 2/3 cup sour cream and powdered sugar in large bowl with wire whisk until well blended.
Add whipped topping; mix well. Set aside. Toss strawberries and blueberries in separate bowl; set aside.
Cut out 2 cake circles from cake layer in each pan, using a 6-inch saucepan lid. (You will have a total of 4 cake rounds.)
Place 1 of the cake rounds on large dessert plate; spread with 1/2 cup of the whipped topping mixture. Top with 1/2 cup of the berry mixture and an additional 1/2 cup of the whipped topping mixture.
Cover with a third cake round. Repeat layers of whipped topping mixture and berry mixture; top with remaining cake layer.
Spread top with remaining whipped topping mixture.
Garnish with fresh mint and additional berries, if desired. Refrigerate until ready to serve. Store leftover dessert in refrigerator.
