Preheat your oven to 375°F. Grease two 9-inch round cake pans and line the bottoms with parchment paper.
In a large mixing bowl, whisk together the flour, sugar, brown sugar, cocoa powder, baking powder, baking soda, and salt.
Add eggs, buttermilk, oil, and vanilla extract to the dry ingredients. Beat on medium speed until well combined.
Carefully stir in the hot water until the batter is smooth. The batter will be thin.
Divide the batter evenly between the prepared pans. Bake for 25-32 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cakes to cool in the pans for about 10 minutes, then remove them to wire racks to cool completely.
In a medium saucepan, combine evaporated milk, sugar, egg yolks, butter, and vanilla extract.
Cook over medium heat, stirring constantly, until thickened and golden brown, about 12 minutes.
Remove from heat and stir in shredded coconut and chopped pecans. Allow the frosting to cool completely.
Place one cake layer on a serving plate. Spread about half of the frosting on top. Repeat with the second cake.
Slice and serve.