Preheat the oven to 350°F (175°C). Grease and flour a 9×13-inch sheet pan.
In a large bowl, whisk together the flour, sugar, baking soda, salt, and cocoa powder until well combined.
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 stir until combined.
In a separate bowl, whisk together the buttermilk, eggs, and vanilla extract.
Add the buttermilk mixture to the batter and stir until smooth.
Pour the batter into the prepared sheet pan and spread evenly.
Bake for 30 to 35 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, sugar, egg yolks, butter, and vanilla extract.
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 and chopped pecans. Let cool slightly.
Once the cake is done baking, remove it from the oven and immediately spread the frosting evenly over the warm cake.
Allow the cake to cool completely before slicing and serving.