Butter and flour 3 nine inch cake pans. Preheat oven to 350°f.
Grate 4 ounces of the German's chocolate and set aside. Blend the remaining 4 ounces of German's Chocolate with the boiling water in a small bowl or measuring cup; set aside to cool slightly.
In a large bowl whisk together and blend well the flour, cocoa powder, baking powder, and salt. Set aside.
With electric mixer, blend 1 1/2 sticks butter, sugar; add egg yolks one at a time blending well after each addition. Ad vanilla and continue to blend. Add mashed potatoes, beat in. Pour in chocolate-water mixture and blend well. Blending slowly at first and speeding up as additions are made, alternately add dry ingredients and buttermilk 1/3 at a time. End with dry ingredients.
In a glass, copper, or stainless steel bowl beat three egg whites to form stiff peaks. Gently fold egg whites and grated chocolate into batter to blend completely.
Distribute batter in three prepared pans as evenly as possible. Bake 20 to 25 minutes at 350°f. Remove when done. Cool on a wire rack.
Remove 1/2 cup of the toasted pecans and 2 tablespoons of the toasted coconut and set aside for garnish.
In a sauce pan on medium heat, mix evaporated milk and 2 1/4 sticks butter. Whisk and melt until thoroughly blended. Add sugar and whisk to blend and cook down the sugar. Whisking briskly so as not to cook the egg, add egg yolks one at a time. Add vanilla and mix well. Cook over low-medium heat for about 12 minutes to cook it down and until it has a rich caramel color. Cool slightly. Stir in the coconut and toasted pecans; blend well.
Finishing cake: Stack layers, using a thin layer of frosting as filling. Frost entire cake. Use remaining pecans, press evenly into sides of cake all the way around. Place pecan halves around edge of top. Place coconut in the center of the top.