How to Make It
Preheat oven to 350°. Lightly grease 3 (9-inch) round cake pans; line bottoms with parchment paper, and lightly grease paper.
Microwave chocolate baking bars and 1/2 cup water in a large microwave-safe bowl at HIGH for 1 to 1 1/2 minutes or until chocolate is melted and smooth, stirring once halfway through.
Combine flour and next 2 ingredients in a medium bowl.
Beat butter and sugars with a heavy-duty stand mixer on medium speed until light and fluffy, about 3 minutes. Add yolks, 1 at a time, beating just until blended after each addition. Add melted chocolate mixture and vanilla; beat on low speed until blended. Add flour mixture alternately with buttermilk, beginning and ending with flour mixture. Beat on low speed just until blended after each addition.
Beat egg whites at high speed until stiff peaks form; gently fold into batter. Pour batter into prepared pans. Bake at 350° for 25 to 30 minutes or until a wooden pick inserted in center comes out clean.
Remove from oven, and run a knife around outer edge of cake layers to loosen from sides of pans. Cool in pans on wire racks 15 minutes. Remove from pans to wire racks; discard parchment paper. Cool completely (about 1 hour).
Spread Coconut-Pecan Frosting between layers and on top and sides of cake. Garnish, if desired, with chocolate-dipped toasted pecan halves.