Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans or one 9×13 inch pan.

In a large bowl, combine the cake mix, eggs, oil, buttermilk, and vanilla. Beat with a whisk or spoon until well combined and most lumps are gone, about 2 minutes.

Pour the batter into the prepared pans, dividing evenly if using round pans

Bake until a toothpick inserted in the center comes out with a few moist crumbs. For round layers, about 25-30 minutes; for a 9×13 pan, about 30-35 minutes.

Cool in pans for 10 minutes, then remove from pans and cool completely on wire racks before frosting.

For the frosting, combine evaporated milk, sugar, egg yolks, and butter in a medium saucepan. Cook over medium heat, stirring constantly, until mixture thickens and just begins to bubble, about 10-12 minutes.

Remove from heat and stir in vanilla, coconut, and pecans. Let cool completely until thick enough to spread, about 30 minutes.

For a layer cake, spread frosting between layers and on top (traditionally leaving the sides unfrosted). For a 9×13 cake, spread frosting over the top.