Preheat oven to 350º. Lightly grease 3 - 9" round cake pans, line bottom with parchment paper, and grease.
Melt chocolate bar in ½ cup water, stir until smooth.
Combine flour, baking soda, and salt in a bowl.
Beat butter and sugar at medium speed until fluffy. Add egg yolks 1 at a time. Beat until blended after each.
Stir in chocolate mixture and vanilla. Add flour mixture alternately with buttermilk (beginning and ending with flour).
Beat at low speed until blended.
In another bowl beat egg whites until stiff peaks form at high speed. Gently fold into batter. Pour into prepared pans.
Bake for 25-30 minutes, until toothpick comes out clean.
Remove pans from oven. Run knife around edge to loosen from sides. Coolin pans on wire rack 15 minutes.
Remove from pans, remove paper, and cool completely (about an hour).
spread coconut-pecan frosting between layers, on top and on sides.