Chocolate is made from a cocoa bean. What keeps that bean from being considered kitniyos?
There were briefly those who'd heard of this thing called coffee -- it's a "bean", you grind it -- who wondered if it were kitniyot.
This was quickly rejected, I don't recall who it was who'd said "if you ever saw a coffee plant, you'd understand why it's not kitniyos."
Very simply -- all kitniyos, like the 5 grains, are annuals. Coffee and chocolate are perennials.
Just because the English word is 'bean' does not mean that it is kitniyos. A good explanation can be found at What is Kitniyot?
Here is the initial summary of that article. You can read the details there.