Preheat oven to 350 degrees. Coat a 9-inch springform pan with cooking spray. Line bottom with parchment paper, and coat with cooking spray. Dust with flour, tapping out excess. Whisk together flour, baking soda, and salt.
Beat butter and sugar with a mixer on medium-high speed until pale and fluffy. Add eggs, one at a time, beating well after each addition. Beat in vanilla. Reduce speed to low. Beat in flour mixture in 3 additions, alternating with buttermilk. Beat in chocolate. Pour batter into prepared pan, and smooth top.
Bake until a toothpick inserted into center comes out clean, 50 to 60 minutes. Let cool in pan on a wire rack 30 minutes. Remove cake from pan, peel off parchment, and let cool completely on rack. Wrap in plastic, and refrigerate until cold, at least 1 hour (or overnight) to make it easier to slice.
Using a serrated knife, slice cake horizontally into 3 layers. Spread 1/3 of the frosting over bottom layer. Repeat, layering cake and frosting, finishing with frosting. Mound coconut in center, and arrange pecan halves around edge.