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.
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 into the dry ingredients and mix until combined.
Add the buttermilk, eggs, and vanilla extract to the batter. Beat with an electric mixer on medium speed 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 is baking, prepare the frosting. In a medium saucepan, combine the evaporated milk, sugar, egg yolks, and butter.
Cook over medium heat, stirring constantly, until the mixture thickens and turns golden brown, about 10 minutes.
Remove from heat and stir in the shredded coconut, chopped pecans, and vanilla extract. Mix well.
Allow the cake to cool for 10 minutes before spreading the warm frosting evenly over the top.
Let the cake cool completely before cutting into squares and serving.