Unless it's just me falling into some newb vortex, the following is borked...
- New composite shot
- New layer (plane) - colour it so you can see it
- Create circular mask in the middle of the plane
- Keyframe Mask -> Transform -> Scale
- Set first value to 0 - second value automatically sets itself to 0 (the link symbol is activated so this seems pretty normal)
- Move down the timeline a number of seconds
- Change the first value of scale back to 100 - the second value is now 10,000