Preheat the oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan.
In a large mixing bowl, combine the flour, sugar, baking powder, baking soda, salt, and cocoa powder. Whisk until well blended.
Add the buttermilk, vegetable oil, eggs, and vanilla extract to the dry ingredients. Beat on medium speed for 2 minutes until smooth.
Stir in the boiling water until the batter is well combined (the batter will be thin). Pour the batter into the prepared baking pan.
Bake for 30-35 minutes or until a toothpick inserted in the center comes out clean. Allow the cake to cool in the pan for about 15 minutes.
Using the handle of a wooden spoon, poke holes all over the top of the cake. Pour the sweetened condensed milk evenly over the cake, allowing it to soak into the holes.
In a small saucepan over medium heat, combine the coconut and chopped pecans. Stir for about 5 minutes until toasted and fragrant.
Spread the toasted coconut and pecan mixture evenly over the cake. Top with chocolate frosting, spreading it evenly over the coconut and pecans.
Refrigerate the cake for at least 2 hours before serving to allow the flavors to meld.