What happens if you cook chocolate pudding too long on a double boiler? I am having some troubles getting my pudding consistency right.

The kind of pudding I am trying to make is of just milk, cream, sugar, cocoa and cornstarch. The double boiler I use is a small pot in a slightly bigger pot, filled with water.

Until now my pudding almost always turned out runny, like a thick sauce. Then I had the revelation that this happens when the water level in my big pot drops below the smaller pot, so that only the vapours are transferring heat to it (I am guessing in my case it doesn't work, since there is a gap between the pots and thus the temperature can't escalate much in this way). So I sort of solved this problem by adding more water. This allowed me to get one pudding right (or so I think) but then the next one was runny again. I got pissed and thought I simply hadn't cooked it enough (I usually take it off the stove when I think it is the right thickness, wait for it to cool down and refrigerate overnight). So I put it on the stove again and left it there for a whole lot of time ~50 minutes or more (I have an automatic stirrer to stir it, first fast, then slowly to let the starch coagulate). And THIS time it became so thick I think it can rightfully be classified in a solid body state. There is no hint of runny-ness, but now it is really hard and gooey, when pulled it stretches sort of like taffy or thick gum, it holds a solid shape and you can't "scoop" it with a spoon, instead you only pull it. It is also a darker color than usual, so I am wondering if I might have burned it or just added too much starch or sugar (this time I was experimenting with more sugar)?

All I knew, the double boiler prevents any burning (in my case there are no burn marks, but maybe the milk got burned/heated too much?) and cornstarch, once heated to the sufficient temperature, would just thicken and further heating it wouldn't change anything, unless it is TOO much heating.

My recipe is:


cocoa 150ml
sugar 500ml
cream 50ml
cornstarch 100ml
milk 800ml


(I measure in ml, don't ask)

So, if nothing was burned or heated too much, it could be that roughly a third of the entire mixture is sugar, while only half is milk and maybe 1/16th of the entire mixture's volume in cornstarch is too much? I have never gotten a definite valid correct sample with this recipe to be able to compare to anything and it will be some time before I can experiment again.
Try using a heat diffuser for your hot plate/burner. You can get them quite inexpensively on Amazon and other places.