"UV dye oil leak detection kit". Available on Amazon. Check the reviews.

Obviously, do the UV dye test.

Most likely it is either the valve cover or the valve cover gasket or both. 

BMW and many other makes switched to plastic valve covers, which over time and heat cycles, will eventually wear. Likewise with the gasket. 

Something to consider is to change out both, even if only one is the culprit, because they both seem to eventually fail. (Specifically the PCV or some sort of valve built into the valve cover.)

The labor is intensive to replace a gasket and the valve cover. It’s not your ordinary engine valve cover that you can plop on and off in 5 minutes. So many things need to come out in order to gain access. That’s why it may be worth considering to do both at the same time. 

But do the test first to confirm.