FOR CAKE: Preheat oven to 350 degrees. Lightly grease 3 (9") round cake pans.
Microwave chocolate baking bars and 1/2 cup water in a large microwave-safe bowl at HIGH for 1 to 1.5 minutes or until chocolate is melted and smooth, stirring once halfway through.
Combine flour and next 2 ingredient in a medium bowl.
Beat butter and sugar at medium speed with an electric mixture until fluffy. Add egg yolks, 1 at a time, beating just until blended after each addition. Stir in chocolate mixture and vanilla. Add flour mixture alternately with buttermilk, beginning and ending with flour mixture. Beat at 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 degrees for 25 to 30 minutes or until a wooden pick inserted in center comes out clean. Remove from oven, and gently 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. Cool completely (about 1 hour). Spread with Coconut-Pecan Frosting between layers and on top and sides of cake.
FOR FROSTING: Cook evaporated milk, sugar, butter, and egg yolks in a heavy 3 quart saucepan over medium heat, stirring constantly for 3 to 4 minutes or until butter melts and sugar dissolves. Cook, stirring constantly, 12 to 14 minutes or until mixture becomes a light caramel color, is bubbling, and reaches a pudding-like thickness.
Remove pan from heat; stir in coconut, vanilla, and pecans. Transfer mixture to a bowl. Let stand, stirring occasionally, 45 minutes or until cooled and spreading consistency.