https://www.youtube.com/watch?v=qpF5B_jHZrw (Adam Ragusea, "How to give brownies a smooth, glossy top") which is where I remembered this from turns out to have been primarily about the appearance of the top - as part of his series of "change two dozen variables one at a time to quantify them properly" baking series (the chocolate chip cookie one is one of the other classics of the form.) (Though I may have it confused with one his other videos since he spends more time on dissolving sugar than boxed mixes here...)