*Confused.* I do not follow.
We have 2 facts: 1 it was a simple dish and despite that (in other words "but") 2 she prepared it exceptionally well.
I understood it as praising her cooking skills and Catherine still giving it her all in terms of skill despite the simplicity of the dish on its own. You'd get the same opposition by breaking the structure into 2 sentences and using "However, ...".
I'd get the meaning of "It trivially easy to make, especially for someone with her cooking skills.", if the quoted text had "and" instead of "but".
Then again, maybe it's the influence of my mother tongue showing here - it strictly requires a comma after every clause.
Either way, I'm genuinely at loss, so would you mind explaining in more detail?