I have a couple recipes for this too...
The first is what I started with, Dave Kodama's -
4-C Sake (Rice Wine)
1-C Mirin (Sweet Rice Wine)
1-C Shoyu (Japanese Soy Sauce)
1-t Hondashi (Japanese Soup Base)
But the real kabayaki/nitsume sauce is a heavily reduced stock made from the heads and spines of the Eels that are filleted. The traditional recipe (rarely seen in restaurants) involves filling the base of a pot with eel heads/bones, and adding (to cover) 7 parts water, 1 part soy sauce, 1 part mirin, 1 part sake. You then add sugar to sweeten. Seeing as I don't have eel heads lying around, I came up with the following recipe...
2-C Dashi (A Japanese Stock)
I'm not a big fan of mirin. To me it seems like cheap sake with corn syrup added to it, so I omit it for just plain good sake and sugar. A good homemade dashi takes minutes to make with bonito and kelp, and is an excellent substitute for the eel stock. It has a delicate yet intoxicating aroma and flavor (it's also the base of any good Miso soup). To make the nitsume, simply combine the ingredients in a saucier and reduce over medium-high heat to a thin syrup (it will thicken considerably as it cools).
I usually stock a couple packs of Unagi in my freezer. I get it pre-charcoal grilled and frozen from Catalina Offshore Products for about $5/eel. I use to eat the stuff like crazy, but now I only keep it for when non-sushi eaters come over and want to try some sushi for the first time. It usually goes over better than serving them a battleship of uni...
Making the rice is actually the longest and most drawn out process...