PREHEAT oven to 350°F. Cover bottoms of 3 (9-inch) round cake pans with wax paper; grease sides of pans. Microwave chocolate and water in large microwaveable bowl on HIGH 1-1/2 to 2 min. or until chocolate is almost melted, stirring after 1 min. Stir until chocolate is completely melted.

MIX flour, baking soda and salt; set aside. Beat butter and sugar in large bowl with electric mixer on medium speed until light and fluffy. Add egg yolks, 1 at a time, beating well after each addition. Blend in melted chocolate and the vanilla. Add flour mixture alternately with the buttermilk, beating until well blended after each addition.

BEAT egg whites in small bowl with electric mixer on high speed until stiff peaks form. Gently stir into batter. Pour evenly into prepared pans.

BAKE 30 min. or until toothpick inserted in centers comes out clean. Immediately run small metal spatula around cake layers in pans. Cool in pans 15 min.; remove layers from pans to wire racks. Remove and discard wax paper. Cool cake layers completely. Spread Coconut-Pecan Filling and Frosting between cake layers and onto top of cake.