How to make a simple "That's all folks!" effect in 2D?

At the end of some Loony Tunes cartoons, there's the "shrinking hole" effect where the screen transitions to black, except for a circular window that shrinks down to reveal only the character's face, or eye, or whatever.

I've tried creating a sprite that's all black, with a transparent circular hole cut out somewhere - then starting it off scaled so that the circular area is greater than the screen-size. On some trigger event, I then scale it down to 1-1.

This works, but it's clumsy, and the sprite-size I need to meet the starting and end-requirements is frankly, silly.

What would be neater would be an XOR circular layer I could place over the top of the whole screen, which would reveal all the content beneath it - then as I scale it down (to do the zoom), a smaller and smaller part of the screen would be visible.

A similar effect could be used to illuminate elements of a scene under 'spotlight' or 'torch' conditions.

So, the question is - how to create an object (or set of objects) that 'reveals' what's beneath it, while keeping any other objects masked out of the scene?