Tantrums & aggression- how to deal

My darling daughter has been throwing tantrums from time to time, but as of the last few weeks, she started slapping herself in the head during her fits of frustration. It's the most bizarre, and scary, thing. She's developmentally doing really well, so I'm not concerned about some underlying issue. I think it's just behavioral. Any suggestions for dealing with this kind of behavior? I tell her in a calm voice "no" and things like "soft" and "be nice" but it seems to be her new go-to move when she doesn't get what she wants. Today i tried ignoring it for the first time suspecting she was attention seeking and she eventually stopped, but only after being distracted. I'd prefer to address it directly BC I don't want the behavior to progress. I know some kids bang their heads, etc. Any advice would be great.