Make batter: Preheat oven to 350 degrees F. Using a small brush, lightly coat two 9-inch cake pans with softened butter or vegetable-oil cooking spray. Dust with flour and tap out any excess. Set aside. In a medium heat-proof bowl, pour boiling water over German’s Chocolate.
Stir until smooth and set aside. In another medium bowl, combine flour, cocoa, baking soda, and salt. Set aside. In a large bowl, using a mixer set on medium-high speed, beat sugar and butter until very light, 1 to 2 minutes.
Add egg yolks, one at a time, until well incorporated. Reduce mixer speed to low and add chocolate mixture and vanilla. Add flour mixture by thirds, alternating with buttermilk and ending with dry ingredients. Thoroughly clean mixer beaters. In a medium bowl, beat egg whites to soft peaks.
Use a rubber spatula to gently stir a half cup beaten whites into batter. Fold remaining whites into batter.
Bake cake: Divide batter equally between pans and spread evenly. Bake on middle rack of oven until a tester inserted in center of each cake layer comes out clean, 30 to 35 minutes. Cool in pans on a wire rack for 15 minutes.
Use a knife to loosen cake from sides of pan and invert onto wire rack to cool completely.
Finish cake: Place 1 layer on a cake plate and top with 1/3 of our Coconut-Pecan Filling. Repeat with second and third layers and remaining filling.
Serve or store in an airtight container at room temperature.