Makes 18 servings at 71¢ each. Prep: 45 minutes. Bake: at 350° for 20 to 25 minutes. Cook: 12 minutes.
Heat oven to 350°F. Grease and flour three 9-inch-round cake pans; tap out excess flour.
Combine half the chopped chocolate and 1/2 cup water in microwave-safe bowl. Microwave at full (100%) power 50 seconds; stir until melted. Or melt in saucepan.
Combine flour, cocoa powder, baking powder and salt in bowl.
Beat together butter and sugar in large bowl until smooth and creamy. Beat in vanilla, egg yolks and mashed potatoes. Beat in melted chocolate, beating until blended.
Beat in flour mixture alternately with buttermilk in 3 additions, beginning and ending with dry ingredients and beating 45 seconds after each addition.
Beat egg whites in clean medium-size bowl with clean beaters until stiff, but not dry peaks form. Fold beaten egg whites and the remaining chopped chocolate into the cake batter until no streaks of white remain. Scrape the batter into prepared cake pans, dividing equally.
Bake in 350°F oven for 20 to 25 minutes or until wooden pick inserted in centers comes out with a few moist crumbs on it. Cool cakes in pans on wire racks 10 minutes; turn cakes out onto wire racks to cool completely.
Combine sugar, butter, the 2 cans of evaporated milk, egg yolks and vanilla in a large heavy saucepan. Cook over medium heat, stirring constantly with wooden spoon, for 12 minutes or until butter is melted, sugar is dissolved and mixture turns golden. Scrape into large bowl. Stir in 2-1/2 cups of the pecans and the coconut. Let stand, stirring occasionally, until the frosting is lukewarm and a good spreading consistency.
Place one cake layer on serving plate; spread top with 1 cup frosting. Repeat with remaining layers. Frost sides of cake with 3 cups frosting.
Press the remaining chopped pecans around sides of cake. Decorate cake with pecan halves and toasted coconut.