Preheat oven to 350F. Microwave chocolate and butter in large microwaveable bowl on HIGH 2 min. or until butter is melted. Stir until chocolate is completely melted. Stir in sugar.
Add eggs, 1 at a time, beating with electric mixer on low speed after each addition until well blended.
Add vanilla; mix well.
Add 1/2 cup of the flour, the baking soda and salt; beat until well blended.
Add remaining 2 cups flour alternately with water, beating until well blended after each addition.
Pour evenly into 2 greased and floured 9-inch round cake pans.
Bake 35 min. or until toothpick inserted in centers comes out clean. Cool in pans 10 min.; remove from pans. Cool completely on wire racks.
Spread with your favorite frosting, if desired. (NOTE: Nutrition information is for unfrosted cake.)