HEAT oven to 350°F.
MELT 6 oz. chocolate as directed on package; cool. Meanwhile, beat dressing, brown sugar and 1-1/2 tsp. vanilla in large bowl with mixer until blended. Beat in eggs and melted chocolate. Combine flour, baking soda and salt. Add to chocolate mixture alternately with water, mixing well after each addition.
POUR into 2 parchment paper-lined 9-inch round pans sprayed with cooking spray.
BAKE 30 to 35 min. or until toothpick inserted in centers comes out clean. Cool 10 min.; remove from pans. Immediately remove parchment paper. Cool cakes completely on wire racks.
MEANWHILE, microwave remaining chocolate and butter in large microwaveable bowl on HIGH 1 to 1-1/2 min. or until butter is completely melted, stirring every 30 sec. Stir until chocolate is completely melted. Cool.
ADD powdered sugar, milk and remaining vanilla to butter mixture; beat until blended. Use to fill and frost cake layers.