Combine the dry ingredients in a coffee mug and mix thoroughly. Then stir in the milk, oil, vanilla, and water. Add in as many Kit Kat pieces as you like and mix. Place in microwave and cook for 1 minute and 45 seconds. Let cool for 5 minutes and spread chocolate frosting on top. You can also sprinkle on any extra Kit Kat pieces if you have leftovers. Dive in and enjoy! Want a little something extra? Top this warm cake with a little vanilla ice cream!