I followed this miso recipe and cooked it on a foil lined sheet pan directly under the broiler. Somehow, I only succeeded in burning the excess marinade on the foil, whereas all the marinade coating the fish retained its light brown miso color. Any ideas as to what could have prevented it from browning properly? 
I've made that recipe before.  Lopez-Alt's assertions to the contrary, getting a good browning on the miso sauce is actually highly dependent on having a good, pre-heated broiler and having the fish exactly the right distance away.  The first time I made it, I had the same result as you; the fish was fully cooked but the sauce hadn't browned. Here's things you can do to make browning better with this, but you'll have to tinker around and try various ones to get the combination that works in your oven:  Precook the marinade, reducing it by 1/3 and making it thicker Add 2 Tbs more sugar do the marinade Pre-heat your broiler making sure that it's very hot before you put the fish in Test distances of 4cm to 10cm from the broiling element/flame Make sure the sauce isn't thicker than 3mm on top of the fish Finish it with a blowtorch  Good luck with this.  At least it tastes delicious regardless of whether it's browned or not. 