Cashmere, hooray for Christopher!!!
Place the chocolate chips in a microwave-safe bowl. Pour the sweetened condensed milk over it, and give it a stir. Minute and a half on high, stir again to blend the melty choc. into the milk. Another minute and a half on high (time depends on microwave). Add the vanilla and stir unti smooth. You can also add peanut butter, nuts, coconut, or any thing else you like at this time, for an individual touch. Pour mixture into greased dish, and place in refrigerator to cool.
No one who has not made fudge in the microwave will believe you did not slave over the stove for ages.
I've never tried this specifically, but I bet it would work atop a stove too, because although the order and method differ, the ingredients are the same as my topping for what I like to call Angelus' Leather Pants Brownies, but which the mundane would identify as fudge topped brownies.
I make my favorite (of the moment) brownie recipe, or brownies from a box if pressed for time (I think Betty Crocker's chocolate tastes more like chocolate than Duncan Hines).
During the last 5 or so minutes of baking, I combine the chocolate chips and can of sweetened condensed milk in a sauce pan, over very low heat, and stir constantly, 'til the chocolate is all melty.
I lost the recipe a long time ago, so I don't measure, and I change the order every time, but it never fails to turn into fudge, and pretty much goes like this:
Once the chocolate is melty, add some vanilla (usually a tsp or two--I don't measure), some salt (I think a 1/2 tsp, and sometimes I put this in at the beginning, when I'm melting the chips in the sweetened condensed milk). Remove from heat, and stir until smooth, and immediately pour on top of the brownies, which are just coming out of the oven.
It's more important that the fudge mixture is poured onto the brownies as soon as the fudge mixture is ready, than it is that the brownies are fresh out of the oven. The mixture always turns into fudge on top of the brownies, so I don't know why it wouldn't turn into fudge if you poured it into a buttered pan.