I don't usually recommenced the use of silly-cone.
However to verify it is the valve cover gasket, get both the mating surfaces super clean, don't even need to be completely flat.
Put a 1/4 inch bead of the silly stuff all the way around on the head.
Let it set up for a good 15 minutes.
Put the cover on and lightly tighten the bolts till the silly cone just peaks out from the edge and let it set overnight.
Once the silly stuff cures, it should not leak.
If it does, then as mentioned above, it's elsewhere.