Cover bottoms of 3 (9-inch) round pans with parchment; spray sides with cooking spray. 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.

Beat egg whites in small bowl with mixer on high speed until stiff peaks form; set aside. Combine flour, baking soda and salt. Beat butter and sugar in large bowl with mixer until light and fluffy. Add egg yolks, 1 at a time, beating well after each. Blend in melted chocolate and vanilla. Add flour mixture alternately with buttermilk, beating until well blended after each addition.