I believe the phrase you are probably thinking about is 'once more' and not what's more. If you mean once more, you are asking that person to do something again. This is not used the same way as also. This may sound weird, but think of also when you want to add something. For example, I ask you to go to the store. I forget that I don't have enough gas in the car so I would say to you, "Can you also stop to get some gas before you go to the store? Does that make sense? I hope this helps. If not, let me know. :)

It would work in informal writing. It isn't exactly the same as 'also' though. 'Also' introduces a new piece of information, but 'what's more' is something that has a higher level of importance to you. 'Cars are useful. They are fast, but what's more is that they let you take many things with you, even friends!' 'Cars are useful. They are fast. Also, they let you take many things with you, even friends!' In the first example, we understand that you place being able to take more things with you as being more important. In the second example the two pieces of information are treated as the same level of importance.