Preheat oven to 350 degrees F (175 degrees C). Grease and flour 3 8-inch cake pans and line bottoms with parchment paper.
Place the chunks of chocolate into the boiling water and stir until the chocolate has melted; set aside to cool. In a large bowl, beat 1 cup of softened margarine with 2 cups of sugar with an electric mixer until light and fluffy. Stir in 4 egg yolks, one at a time, beating well after each addition. Beat in 1 teaspoon of vanilla extract and the chocolate-water mixture until thoroughly blended.
Sift the flour, baking soda, and salt into a bowl. Beat the flour mixture into the liquid ingredients, alternating with buttermilk, in 3 or 4 additions. Beat until batter is smooth, about 1 minute.
In a metal or glass bowl, beat the reserved egg whites until they hold stiff peaks when the beaters are lifted straight up. Gently fold the beaten egg whites into the batter, keeping as much volume as possible. Pour the batter into the prepared cake pans.
Bake in the preheated oven until the cakes have browned lightly and a toothpick inserted into the center of a cake comes out clean, 35 to 40 minutes. Allow the cakes to cool in pans before removing.
To make frosting, pour the evaporated milk into a large saucepan, and mix in 1 cup of sugar, 3 egg yolks, 1/4 cup of margarine, and 1 teaspoon of vanilla extract. Bring to a boil, reduce heat to medium, and cook until thickened, stirring constantly, about 12 minutes. Remove from heat, and mix in the coconut and pecans, beating the frosting until cooled and spreadable. Frost and fill the cake with the coconut frosting.
Instead of 3 round layers, you can make 2 9x9x2-inch square cakes. Bake square cakes 40 to 45 minutes.
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