HEAT oven to 350°F.
MICROWAVE chocolate and 3/4 cup butter in large microwaveable bowl on HIGH 2 min. or until butter is melted. Stir until chocolate is completely melted and mixture is well blended.
ADD whole eggs, sour cream and 1 tsp. vanilla; beat with mixer until blended. Add combined flour, sugar, baking soda and salt; mix well. Pour into 13x9-inch pan sprayed with cooking spray.
BAKE 30 to 40 min. or until toothpick inserted in center comes out clean. Cool completely.
COOK sugar, evaporated milk, egg yolks, 6 Tbsp. butter and remaining vanilla in large saucepan on medium heat 6 min. or until thickened, stirring occasionally. Remove from heat; stir in coconut and nuts. Cool until thickened to spreading consistency, stirring occasionally. Spread over cake.