What produces more crema when extracting espresso, darker or lighter roast?
What produces more crema when extracting espresso, finer or coarser coffee grind ?
why I'm getting a very thin layer of crema when pulling a SINGLE shot, although the extraction time is 20 sec, the pressure is correct as per the manufacturer of the espresso machine (Breville Barista) I have, and the coffee beans were roasted before two weeks?! what is wrong?
The CO2 stuck within coffee grounds produce the crema. At least, it is the main substance that forms the crema along with other minor components.
This is a very classic problem -and thus a question.
If you consider roasting, darker roasts have more crema as they contain more CO2 as the product of Maillard reaction. Here is a proposed previous post to be read. You may also want to check this previous post as it may lead to more reading on other metrics you may consider.
Grind has some effect, but limited. You may see the factors that effect crema production in espresso in this proposed previous post to be read.
As you have mentioned, time has a crucial importance. I think this should be your main consideration and you should focus on fixing this first. Basically, fresh coffee has more crema, always. You can read a lot more in this previous post.