Adjust your oven rack to the center and preheat the oven to 350°F. Prepare two 9-inch round cake pans by greasing them thoroughly, lining the bottoms with parchment paper, and greasing and flouring both the paper and the pan sides. Set aside.

In a small mixing bowl, whisk together the flour, baking soda, and salt. Set aside.

In another bowl, combine the cocoa powder and chopped chocolate. Pour the boiling water over the mixture, cover the bowl, and allow it to sit undisturbed for 5 minutes. Afterward, whisk until smooth and glossy, then let the mixture cool to room temperature.

In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, granulated sugar, and brown sugar together on medium speed until the mixture is light, fluffy, and creamy—about 3 minutes. Add the eggs one by one, beating well after each to ensure a uniform mixture.

Add the vanilla extract and continue beating for another minute until the mixture is voluminous and pale. Scrape down the sides of the bowl, then add the cooled chocolate mixture and beat for another 30 to 45 seconds until fully incorporated. Scrape down the bowl again to ensure even mixing.

With the mixer on low, add the dry ingredients and sour cream alternately in three parts, beginning and ending with the flour mixture. Mix just until the batter is smooth and combined.

Divide the batter evenly between the prepared pans and smooth the tops.

Bake for 30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. While the cakes are baking, begin preparing the coconut pecan frosting.

Once baked, transfer the pans to a wire rack and cool for 10 minutes. Carefully remove the cakes from the pans, discard the parchment paper, and allow the layers to cool completely on the rack.

When fully cooled, slice each cake horizontally to create four even layers. Place one layer on your serving plate or cake stand, spread approximately 1½ cups of frosting over the surface, and repeat with the remaining layers, using about 1 cup of frosting between each. Leave the sides of the cake exposed. Slice and serve.

In a medium saucepan, whisk together the heavy cream, granulated sugar, egg yolks, and salt. Place the pan over medium heat, stirring occasionally until the mixture begins to simmer gently and thickens slightly.

Remove from heat and immediately stir in the butter, vanilla extract, toasted pecans, and toasted coconut. Stir until fully incorporated. Allow the frosting to cool to room temperature, during which it will thicken to a spreadable consistency.