This is the easiest cake in the world. Really. And it's vegan, should you care about that. And a responsible child can do the whole thing on her own (or his, whatever), so long as you handle the oven bits. It's also tasty. You CAN frost it - it's certainly nice frosted - although I'm prone to just devouring the whole thing hot straight from the oven.
Get out your square 9" cake pan. Don't grease it.
1 1/2 cups flour
3 tablespoons cocoa
1 teaspoon baking soda
1 cup sugar
1/2 teaspoon salt
5 tablespoons cooking oil
1 tablespoon vinegar
1 teaspoon vanilla
1 cup cold water (or cold coffee, if you want a mocha cake)
Mix your flour, soda, sugar, cocoa and salt right in the pan. Make three little wells in the mixture. Into one, pour the oil; into the next, the vinegar; into the next, the vanilla. Now pour the cold water over it all and stir it together - you'll see greyish swoops as the soda and vinegar react! Bake it at 350 for half an hour.