LAST EDITED ON 05-24-12 AT 08:48 PM (EST)Thanks for the tips, tkit.
I had set the timer for eight minutes, but since it was a half batch, I thought it may need less time and sure enough, at about six minutes it started to show that darker yellow at the bottom of the pan, so I took it off the double boiler and shut off the timer. Then I thought that maybe the yellow was the custard forming, so I put it back on for a little while, maybe another minute. That was my mistake. I guess I would have done better with some idea of what signs to look for instead of just a time, which can vary. I am still not sure how to look at it and know it's done because obviously it should be taken off the heat before it starts to turn yellow. It never looked like a custard to me; it was pretty runny even when overcooked.
I have a Microplane and love it, but it's the size for making zest strips like you've used for your garnish or for curly strips of chocolate or parmesan. I also have a quality mini box grater that works extremely well for citrus zest, but it has several sizes and I just used the wrong one. When it wears out, I'll probably replace it with a smaller Microplane. They really are terrific.
I think orange might be good, but I'd probably add a bit of orange extract to ramp up the flavor a bit and lessen the sugar. I think lemon or lemon-lime would both be great variations. I noticed Silver Palate suggested a lemon version. Many delicious possibilities!