Preheat oven to 375°F. Grease 3 (9-inch) cake pans; line with parchment; grease and lightly flour.
Process flour, 1 cup nuts, baking soda and 1/2 tsp. salt until nuts are ground. In a bowl, whisk bittersweet chocolate and 1/2 cup boiling water until smooth.
Cream 2 sticks butter and 1 1/4 cups sugar until fluffy. Add 4 yolks one at a time, blending after each. Stir in chocolate mixture.
Mix sour cream and 1 tsp. vanilla. Add flour and sour cream mixtures alternately to chocolate mixture, beginning and ending with flour. In a separate bowl, beat egg whites until frothy. Add 1/4 cup sugar 1 Tbsp. at a time; beat until egg whites hold soft peaks. Fold whites into batter in thirds. Divide batter among pans. Bake until cake springs back lightly, 25 to 30 minutes. Cool on a wire rack for 5 minutes before inverting cakes onto rack.
Whisk 3/4 cup sugar, evaporated milk, 1 stick butter, 4 yolks and 1 tsp. vanilla in a pan over low heat until smooth. Cook filling, stirring constantly, until thickened and just boiling, 10 minutes. Remove from heat and stir in coconut and 1 1/4 cups pecans.
Bring cream, 1 1/4 cups sugar and corn syrup to a boil, stirring to dissolve sugar. Reduce heat and simmer, stirring, 5 minutes. Remove from heat, add unsweetened chocolate and 1 stick butter; stir until smooth. Add 1 tsp. vanilla and 1/4 tsp. salt; pour into a bowl and set in a bowl of ice water to cool. Beat until frosting is shiny and thick.
Place one cake layer on a plate; spread with half of filling. Top with second layer and remaining filling. Top with third cake layer. Frost cake; garnish with additional coconut and pecans, if desired. Chill for 30 minutes.