Preheat the oven to 350 degrees. Coat three 9-inch cake pans with cooking spray; line with parchment paper rounds and coat the paper with spray.
In a small bowl, microwave the chocolate and 1/2 cup water at 50 percent power in 15-second increments, stirring in between, until the chocolate is just melted. In a medium bowl, whisk the flour, cocoa powder, baking soda and 1 tsp. salt. In a large bowl using an electric mixer, beat the butter on medium speed until pale yellow, 1 to 2 minutes. Add the sugar and beat until light and fluffy, 1 to 3 minutes. Add the egg yolks one at a time, beating well after each addition. Beat in the melted chocolate and vanilla. Add the flour mixture in 3 additions, alternating with the buttermilk in 2 additions, starting and ending with the flour mixture.
In a medium bowl using an electric mixer with clean, dry beaters, beat the egg whites on medium-high until firm peaks form, about 4 minutes. Gently fold the egg whites into the cake batter until just blended (it's OK if you see a few white streaks).
Divide the batter between the prepared pans. Bake until a toothpick inserted into the center of each cake comes out clean, about 30 minutes. Transfer the cakes in the pans to wire racks and let cool 10 minutes (cakes may fall slightly). Run a knife around the edges of the pans to release the cakes; invert onto the racks. Peel off the parchment and let the cakes cool completely.
In a large saucepan, whisk the egg yolks, evaporated milk and vanilla until blended. Add the sugar and butter. Cook over medium heat, stirring constantly until the mixture turns golden brown and thickens, about 12 minutes. Remove from the heat and mix in the coconut and 1 1/2 cups pecans. Transfer to a medium bowl and let cool to room temperature, stirring occasionally, about 30 minutes.
Place 1 cake round on a large plate or cake stand. Spread one-third of the frosting (about 1 2/3 cups) over the cake. Repeat with remaining cake rounds and frosting, leaving the sides of the cake unfrosted. Top with more pecans.