Preheat the oven to 350 degrees F and grease a 9-inch round cake pan.
In a medium bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt.
In a large bowl, cream together the butter and sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla extract.
Slowly add the dry ingredients to the butter mixture, alternating with the buttermilk. Mix until just combined.
Pour the batter into the prepared cake pan and bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely.
To make the topping, combine the shredded coconut, chopped pecans, evaporated milk, brown sugar, egg yolks, butter, and vanilla extract in a saucepan. Cook over medium heat, stirring constantly, until the mixture thickens.
Once the cake is cool, spread the topping evenly over the top. Let the cake chill in the refrigerator for at least 30 minutes before serving.