Preheat oven to 350°F (175°C). Grease or line a 9×13-inch sheet pan.
In a large bowl, cream together butter and sugar until light and fluffy.
Add eggs one at a time, beating well after each addition. Stir in vanilla extract.
In a separate bowl, whisk together flour, cocoa powder, baking soda, and salt.
Alternate adding dry ingredients and buttermilk to the butter mixture, beginning and ending with dry ingredients.
Slowly stir in boiling water until batter becomes smooth and slightly thin.
Pour batter into prepared pan and bake for 30–35 minutes or until a toothpick inserted in the center comes out clean.
Allow the cake to cool completely on a wire rack.
For the frosting, combine evaporated milk, sugar, egg yolks, and butter in a saucepan. Cook over medium heat, stirring constantly until thickened (about 10–12 minutes).
Remove from heat, add vanilla, coconut, and pecans. Mix well.
Spread the warm frosting evenly over the cooled cake.
Let the frosting set before slicing and serving.
