Preheat the oven to 350°F (175°C). Grease and flour a 9×13 inch baking pan.
In a large bowl, sift together the flour, sugar, baking soda, salt, and cocoa powder. Set aside.
In a medium saucepan, combine the butter and water. Bring to a boil over medium heat, stirring occasionally.
Pour the boiling butter and water mixture over the dry ingredients. Stir until combined.
Add the buttermilk, eggs, and vanilla extract to the batter. Beat with an electric mixer on medium speed for 2 minutes until smooth.
Pour the batter into the prepared baking pan and spread evenly.
Bake for 30 minutes or until a toothpick inserted in the center comes out clean.
While the cake bakes, prepare the frosting: In a medium saucepan, combine evaporated milk, sugar, egg yolks, and butter.
Cook over medium heat, stirring constantly, until the mixture thickens and turns golden brown, about 12 minutes. Remove from heat.
Stir in the shredded coconut, chopped pecans, and vanilla extract. Mix well.
Allow the cake to cool in the pan for 10 minutes, then spread the warm frosting evenly over the cake.
Let the cake cool completely before cutting into squares and serving.