Clean out the loose material and use plaster patch. I would paint the broken edges with latex additive or white glue to improve the bond. Plaster patch dries pretty fast, so you will have to work fast to trowel it level.
You might prefer to use one of the quick dry drywall compounds. They come as a powder to be mixed with water, and they give you a little more time to work. They resemble plaster more so that drywall compound does.