Preheat oven to 350°F. Microwave chocolate and butter in large microwaveable bowl on HIGH 2 minutes or until butter is melted. Stir until chocolate is completely melted. Add sugar; mix well. Add eggs, 1 at a time, beating on low speed of electric mixer after each addition until well blended. Add vanilla; mix well. Add 1/2 cup of the flour, the baking soda and salt; mix well. Add remaining 1-1/2 cups flour alternately with buttermilk, beating until well blended after each addition.

Pour into greased 13x9-inch baking pan.

Bake 50 minutes or until toothpick inserted in center comes out clean. Cool in pan on wire rack. Frost with Coconut-Pecan Filling and Frosting.