Preheat oven to 350°F (180°C).
In the bowl of a stand mixer fitted with the paddle attachment, beat butter and sugars at medium speed until fluffy, 3 to 4 minutes, stopping to scrape sides of bowl. Add eggs, one at a time, beating well after each addition. Beat in vanilla.
In a medium bowl, whisk together flour, cocoa, baking powder, baking soda, and salt. With mixer on low speed, gradually add flour mixture to butter mixture alternately with buttermilk, beginning and ending with flour mixture, beating just until combined after each addition. Stir in melted chocolate until combined.
Spray a 12-cup Bundt pan with baking spray with flour. Spoon batter into prepared pan, and spread into an even layer. Tap pan on a kitchen towel-lined counter several times to settle batter and release air bubbles.
Bake until a wooden pick inserted in center comes out clean, 50 to 55 minutes. Let cool in pan for 15 minutes. Invert cake onto a wire rack, and let cool completely.
Measuring 1¼ inches from bottom of cake, cut cake in half horizontally using a large, serrated knife. Using a cake board or cake lifter, remove top half, and set aside. Spread Coconut-Pecan Filling evenly on top of bottom half. Replace top half. Spread Creamy Chocolate Frosting on top of cake. Garnish with toasted coconut and chopped pecans, if desired. (Refrigerate for 30 minutes to set frosting for a cleaner cut, if desired).
