Preheat oven to 350°F (175°C) and grease two 9-inch round cake pans.
Whisk flour, sugar, cocoa powder, baking powder, baking soda, and salt in a large bowl.
Add buttermilk, oil, eggs, and vanilla then mix until smooth.
Stir in hot water until batter is thin and well combined.
Divide batter evenly into pans.
Bake for 30–35 minutes until a toothpick inserted comes out clean.
Cool cakes completely before frosting.
For frosting cook evaporated milk, brown sugar, butter, and egg yolks over medium heat until thickened.
Stir in coconut, pecans, and vanilla then cool slightly.
Spread frosting between layers and over the top of cake.