PREHEAT oven to 350°F. Place 4 oz. chocolate and 1/2 cup butter in large microwaveable bowl. Microwave on HIGH 1 to 1-1/2 min. or until butter is melted. Stir until chocolate is completely melted. Blend in sugar. Add eggs, 1 at a time, beating on low speed after each addition until well blended. Add vanilla; mix well. Add 1/3 cup of the flour, the baking soda and salt; beat well. Add remaining 1-1/3 cups flour alternately with the water, mixing until well blended. Pour into greased and floured 13x9-inch baking pan.
BAKE 30 min. or until toothpick inserted in center comes out clean. Cool completely in pan on wire rack.
MIX coconut and sweetened condensed milk until well blended; spread over cake. Microwave whipping cream and 2 Tbsp. butter in large microwaveable bowl on HIGH 2 min.; stir. Microwave an additional 30 sec. or until mixture comes to boil. Add remaining 10 oz. chocolate; stir until chocolate is completely melted. Cool until mixture is of desired spreading consistency, stirring frequently. Spread over cake. Let stand until chocolate layer is firm.