Preheat oven to 350°F (175°C). Grease and line three 9-inch round cake pans.
In a bowl, whisk together flour, cocoa, baking powder, baking soda, and salt.
In a separate bowl, beat sugar, eggs, oil, buttermilk, and vanilla until smooth. Gradually mix in the dry ingredients.
Stir in the hot water or coffee until well combined. The batter will be thin.
Divide batter evenly into the prepared pans and bake for 25–30 minutes, or until a toothpick inserted comes out clean.
Allow cakes to cool completely before frosting.
For the frosting: In a saucepan, combine evaporated milk, sugar, egg yolks, and butter. Cook over medium heat, stirring constantly, until thickened (about 10–12 minutes).
Remove from heat and stir in coconut, pecans, and vanilla. Let cool before using.
Assemble the cake by layering coconut-pecan frosting between each cake layer and spreading it generously on top. Optionally frost the sides with chocolate buttercream.