In the interest of full disclosure... I'm going to tell you that I screwed this one up... definitely not a baking success here. But I learned something, and in the end, the cake was really good -- just not really what I was going for.
Dulce de leche is a thick, caramel-like sauce that is made from sweetened, condensed milk. It is supposed to be easy... buy a can of sweetened, condensed milk and cook it until it becomes dulce de leche. Supposedly this works in the oven, boiling on the stove, or even in the microwave. I should have known this would be a bad idea. But to do it on the stove or in the oven takes hours. I found a recipe here that claimed it would work in under 20 minutes in the microwave. That was perfect for me considering I decided to make it less than 4 hours prior to dinner and still had to make the cake.
So this process started out fine, empty the milk into a large bowl (it really does have to be at least 2.5 quarts as I quickly found out) and cover with plastic wrap. Figure out how to adjust the power settings on your microwave and reduce it to 50% power.
Cook the milk for 2 minutes, whisk and recover. Repeat for the next 10-15 minutes. Sounds easy enough. And at this point it actually was working quite well, getting thicker and thicker with each interval.
It all worked well, aside from the bowl screw-up, until I got to the part of the recipe that said - "let cool before packing into a glass jar" - well I left it alone for 20 minutes and when I came back it was a rock hard mess stuck to the bowl. I couldn't budge it with a spoon. I didn't have another can of sweetened, condensed milk and the cake was already in the oven so I had to try to save it.
My solution: add a little heavy cream, about a tablespoon full at a time and microwave for a few seconds to reheat it. As it re-melted a little bit, I was able to start incorporating the cream and it became a (slightly thinner) sauce again, however after I got tired of whisking there were still a few lumps. Oh well - it was still good, but it definitely wasn't the dulce de leche I was going for when I started. Next time, I'll go back to the old-fashioned method and bake it. So much for short cuts.
For the cake - I used this yellow cake recipe from Mark Bittman - just baked in 2 small oval pans to make a much smaller cake. I filled the cake with the dulce de leche and poured the extra over the top and then frosted the whole thing with a whipped cream frosting. Despite the troubles, the cake was delicious and I'll let you know next time how the dulce de leche in the oven comes out.
FYI - you can also buy prepared dulce de leche in some grocery stores and at Williams-Sonoma.