How to make wooden hollows?

[Mavrik]

My wife has asked me to make a wooden board with many hemisphere hollows - like those in an ice cube tray:

The idea is that she'll keep pairs of earrings in each "hollow".
Any idea how I could make these hollows in wood?

 

[Deansocial]

Router

 

[sgryd]

I've seen a drill bit for egg cup drilling. Might fit your needs, but I don't know where to find one.

 

[AlexThePalex]

Half round bit in the router.

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