What makes her laugh are dirty jokes.
The meaning: We hear her laughing right now. We both know that she is laughing right now because she has just heard some jokes. I want to say to you that in my view the jokes she is laughing to are dirty.
What makes her laugh is dirty jokes.
The meaning: When she hears dirty jokes, she usually laughs.
[What make her laugh] are dirty jokes. (WRONG)
..this sentence is wrong because the verb make is inside the content clause (in brackets) and should correspond to the subject word what.
The verb be has a "specifying" use:
What I need right now is a good sleep. (I specify what I need right now)
...and an "ascriptive" use:
What I'm doing right now is interesting. (I describe what I'm doing; I'm ascribing some property to the content clause "What I'm doing right now")
Imagine I've bought some jewels - several of them. And suddenly an expert says:
What you have bought is fake. (ascriptive use)
..this sentence would be equal in meaning to
What you have bought are fake jewels. (the experts says to me, holding the jewels in his hand. He describes the jewels. He ascribes some property to them: the property is "fake")
But imagine I bought something while I was very drunk. I wake up and cannot understand why my bank account has diminished. I call my friends with whom I was drinking, and inquire what did I use the money for. They laugh and reply:
"What have I bought, guys?"
"What you have bought is fake jewels. A wagonload of 'em. We tried to dissuade you, but it was impossible." (the specifying use of the verb be)