Preheat the oven to 350°F (175°C). Grease and flour a 9×13 inch sheet 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 hot butter and water mixture over the dry ingredients. Stir until well combined.
Add the buttermilk, eggs, vanilla extract, and vegetable oil to the batter. Beat with an electric mixer on medium speed for 2 minutes until smooth.
Pour the batter into the prepared sheet pan and spread evenly.
Bake for 30 minutes or until a toothpick inserted into the center comes out clean.
While the cake bakes, prepare the frosting. In a medium saucepan, combine evaporated milk, granulated sugar, egg yolks, butter, and vanilla extract.
Cook over medium heat, stirring constantly, until the mixture thickens and turns golden brown, about 12 minutes.
Remove from heat and stir in shredded coconut and chopped pecans. Mix well.
When the cake is done, remove it from the oven and immediately spread the warm frosting evenly over the top.
Allow the cake to cool completely before slicing and serving.