Make the Cake
Preheat your oven to 350°F. Grease two 9-inch cake pans and line the bottoms with parchment paper.
In a large bowl, combine flour, cocoa powder, sugar, baking powder, baking soda, and salt. Whisk until evenly mixed.
Add the eggs, buttermilk, oil, and vanilla extract. Mix on medium speed until smooth.
Slowly pour in the boiling water. The batter will be thin—this is correct.
Divide the batter evenly between the pans. Bake 28–32 minutes, or until a toothpick comes out with a few moist crumbs.
Cool in pans for 10 minutes, then remove the cakes and let them cool completely on a wire rack.
Make the Frosting
In a medium saucepan over medium heat, whisk together evaporated milk, sugar, egg yolks, and butter.
Cook, stirring continuously, until thickened (10–12 minutes).
Remove from heat and stir in vanilla, sweetened shredded coconut, and pecans.
Let cool until thick and spreadable.