1999 Toyota Camry Oil leak around valve cover and smoke on

I have been having issues with my camry leaking oil around the valve cover gasket. I have replaced the gasket twice in the past 8 months and it still leaks down the backside of the engine. When taking the gasket off, the backside looks as though it has been hot and is brittle. The cover has been tightened as it should. I am also getting puffs of smoke on startup, after it has been sitting for a while. I found oil in one of the spark plus areas and replaces the gaskets there as well. I suspect I may need to have the valve ring seals replaced.

Any idea why the valve cover gasket continues to leak. Any ideas of how to fix this issue? Mechanic did put something on/around the gasket to help in sealing, but does not work.