Start by breaking the chocolate bar into small pieces and placing it in a heatproof bowl. Pour the boiling water over it and let it sit for a minute before stirring until completely smooth. Set this aside to cool slightly while you prep everything else. This step gives the cake its deep, rounded chocolate flavor.
Whisk together the flour, baking soda, and salt in one bowl and set it aside. In a separate large bowl, beat the softened butter and sugar until light and fluffy, about 3-4 minutes. Add the egg yolks one at a time, beating well after each addition so the batter stays smooth and glossy.
Blend the cooled melted chocolate and vanilla into the butter mixture until fully incorporated. Add the flour mixture alternately with the buttermilk, starting and ending with flour, mixing just until combined. Overmixing at this stage can make the cake tough, so stop as soon as the streaks disappear.
In a clean, dry bowl, beat the egg whites until stiff peaks form. Gently fold them into the chocolate batter using a rubber spatula, working in batches so you don't deflate all that air. This step is what makes the cake rise tall and stay soft instead of dense.
Divide the batter evenly between three greased and parchment-lined 9-inch pans. Bake at 350°F for 28-30 minutes, or until a toothpick inserted in the center comes out clean. Let the layers cool in their pans for 10-15 minutes before turning them out onto a wire rack.
Make the Coconut Pecan Filling
While the cake cools, whisk the evaporated milk, sugar, egg yolks, butter, and vanilla together in a saucepan over medium heat. Stir constantly for about 10-12 minutes until it thickens into a pudding-like consistency. Remove from heat and stir in the toasted coconut and pecans until well combined.
Once the layers and filling have both cooled to room temperature, place one layer on your serving plate and spread a generous layer of filling on top. Repeat with the second and third layers, finishing with filling on top and leaving the sides bare for that classic rustic look. Let the assembled cake sit for 20-30 minutes so the filling can set slightly before slicing.