Preheat the oven to 350F and grease and line two 8-inch round pans. Stir the milk and vinegar together and let sit for 5 minutes.
In a large bowl, whisk together the flour, sugar, cocoa powder, baking soda, baking powder, and salt.
Add the oil and vanilla to the milk mixture and stir it into the dry ingredients, then stir in the hot water until the batter is smooth - it will be thin, and that is exactly right.
Divide between the pans and bake for 30-33 minutes, until a toothpick comes out clean. Cool in the pans for 10 minutes, then transfer to racks to cool completely.
For the frosting, whisk the cornstarch into 1/4 cup of the evaporated milk until smooth and set this slurry aside.
In a saucepan, simmer the remaining evaporated milk, brown sugar, and butter for 3-4 minutes. Whisk in the cornstarch slurry and cook for 2-3 more minutes until thick like pudding.
Remove from the heat and stir in the coconut, pecans, vanilla, and salt. Let the frosting cool completely - it thickens as it cools.
Place one cake layer on a plate, spread half the frosting over it, top with the second layer, and spread the remaining frosting on top, leaving the sides bare in classic style. Let set for 30 minutes before slicing.
