I'm going to ask my (unemployed) step-daughter to cook as part of her chores
Is cooking something that your step daughter is pretty good at?
Actually no. She hates to cook and I have yet to tell her that I am adding it to her chores.
You should be more straightforward by telling people what your expectations are. If your step daughter does not like cooking then it would be appropriate for you to find her a chore that she does not dislike so much.
No, I'm going to tell her at the last minute.
Well, asking your step daughter to cook as a chore seems like you are taking advantage of her a bit. You may be better off just ordering food if you do not feel like cook, as this will keep the peace between you and your step daughter.