Preheat oven to 375F. Microwave butter, water and 2 oz. chocolate in large microwaveable bowl on HIGH 2 min. or until butter is melted; stir until chocolate is completely melted. Chop remaining 6 oz. chocolate; set aside. Stir in combined flour, 2 cups of the sugar, the baking soda and salt.
Add 2 of the eggs and the sour cream; mix well.
Pour into greased and floured 15x10x1-inch baking pan.
Beat cream cheese and remaining 1/3 cup sugar in small bowl with electric mixer on medium speed until well blended.
Add the remaining egg; mix well. Spoon over chocolate batter; cut through batters with knife several times for marble effect.
Sprinkle with the chopped chocolate.
Bake 25 to 30 min. or until toothpick inserted in center comes out clean. Cool.