In the mixer bowl, cream the remaining 1 1/2 sticks of butter and sugar together until fluffy. Add the egg yolks one at a time, beating well after each addition. Add the melted, cooled chocolate and the vanilla. Mix well.
With the mixer on very low, stir in the flour mixture alternately with the buttermilk. Do this by adding about a third of the flour and slowly stirring it in completely. Then add about half the buttermilk and stir it in. Continue adding flour and buttermilk in this manner, ending with flour. Scrape the sides and bottom of the bowl and stir again. With a long-handled spoon or spatula, fold and stir the beaten egg whites into the batter until the batter is smooth with no visible clumps of whites.
Divide the batter evenly between the prepared pans and bake for 30-40 minutes. Bake on the middle rack of the oven, allowing at least 1/4-inch clearance between the pans and the oven walls. The cake will rise above the pan edges as it bakes but will not spill over and will settle back down as it continues to bake. The cake is done when it begins to pull away from the sides of the pans and springs back to a light touch. Cool layers in the pans for about 8 minutes.
Run a knife around the edges of each pan and turn the layers out onto wire racks that have been sprayed with cooking spray. Cool layers completely before frosting.
To make the frosting, combine the sugar, egg yolks, and evaporated milk in the top of a double boiler. Stir with a wire whisk until the yolks are fully incorporated. Add the butter. Place over simmering water and bring to a boil (see Note below). Simmer for 12-15 minutes longer, stirring constantly, until the mixture thickens. Add the vanilla, coconut, and nuts. Cool.
To assemble the cake, place one layer on a cake stand and spread with frosting. Frost each layer completely, top and sides, as it is added to the cake.
Note: You can also make the frosting in a regular saucepan, but be sure to stir it constantly, as it scorches quite easily. Also, you must use the finely grated fresh or frozen coconut, not canned or shredded, to be able to spread the frosting on the sides of the cake easily.
Shopping Hint: For those cooks who use a lot of sweet baking chocolate, the chocolate used in this recipe can be purchased in bulk online at www.cocoasupply.com. Choose La Equatoriale – Dark Chocolate Coverture. The cost, including postage, is half what you would probably pay in grocery stores. Share the large bar with your friends who bake.