I have been told it’s pure milk fat. Is it? Should I remove it or stir it back in?
It’s a layer of coagulated milk proteins that float to the top.
You can’t stir it back in because it will stay together in clumps and make your hot chocolate nasty.
If you stir the milk while it’s heating, the skin won’t form because the proteins stay distributed throughout the milk.
The above explanation is correct. The way to deal with it is to never let the milk start to boil. Watch around the rim of the pan for small bubles starting. Remove the pan from heat at that point. It’s the standard way to heat milk for cooking. Once the skin forms, you have to remove it.
The Sniglets guy calls it “bleemus”.
Hmm, and I always thought it was distilled essence of “gross.”
Moved to CS.
-xash
General Questions Moderator
It’s not that gross. When dried out and powered it becomes analogous to this.
It’s mostly the texture I find unpleasant.
Why was this moved to CS? Just curious. It doesn’t appear to have anything to do with arts and entertainment.
In India they call it malai, ‘cream’, because they perceive it to have a higher fat content than milk. I think they prepare it carefully so that the the coagulated casein skin remains soft and not overcooked, not rubbery; this allows it to mingle with the cream that rises to the top of unhomogenized milk. In Indian teahouses they add a spoon of this carefully prepared malai to the top of a cup of chai, and it’s considered a delicacy.
But in Japan, the skin of cooked soymilk is highly appreciated. They call it yuba. It is cooked until firm, then toasted and used to wrap sushi. Yumm-yumm. The sushi stand at the local Whole Foods was taken over by a new company last week; when I asked for vegetarian sushi they handed me some wrapped in yummy yuba. It’s crinkly on the outside and chewy on the inside, but very light and delectable.
Yeah this is a pretty straightforward factual question if ever there was one.
A layer of whey protien? Yum! Boy have you changed my mind!
I kind of like it. And on custard? Mmmmm.
Moved back to GQ by public demand 
Sorry, on glancing over the OP I saw just the last part of the question and totally missed the actual question. My mistake.
-xash
General Questions Moderator
Although this has been answered, you can perform your own experiment. The same thing happens even if you use skim milk.
Skim milk would, if anything, have a higher casein content, and grow a thicker skin. That’s how to deal with perceived insults better: drink more skim milk.
Indian teahouse malai is a mixture of softly coagulated casein (think soft boiled egg texture) with milkfat. I think of it as an economical way to extend the cream. It makes the casein much more palatable and creamlike.