Where I come from (Central Pennsylvania), a black cow is a chocolate ice cream root beer float. With vanilla, it is merely a root beer float.

Caution: Please note that, like all floats made with carbonated soda water and ice cream, black cows are highly unstable. If you stir the concoction with your straw or dunk the ice cream with your spoon, you will cause the carbonation to come out of solution, and the contents will froth over the lip of your glass. You do not want to try to get this stuff out of your Armani suit.

Black cows are an order of magnitude superior to other fountain treats. They are, in an age when casual slap-and-tickle is risky, excessive marijuana use is passé, and loud noises are tiresome, one of life's last remaining self-indulgent sins actually worth the price. I had one today.