Under that metal cap there is a rubber grommet that seals the valve cover so oil does not come out of there. Not exactly sure how violently the oil moves inside there but I wouldn't drive the car as is.
Try to remove the metal cap only and try to bolt the acorn nut onto the snapped bolt so you may get a good seal. If you drive around as-is it might vibrate off and fall while you drive.
In order to remove the snapped head bolt you have to remove the valve cover and and use a long tube socket to remove. I believe a 12mm. Once you remove the snapped bolt place a new one in and torque to spec and reassemble the valve cover.
This is what it looks like once the VC is off....
This is just like the bolt you snapped bu in a different spot on the head...
