Anthelle, what happened was you fell into a selling technique called an "assumed close". It's sometimes used in several industries. "Just sign your name on the dotted line and you'll be all set!" It's effective with some people, but others find it obnoxious and develop an immediate reaction to it. You basically set off one of her red flags and she got angry about it.
I haven't heard this term before but from my recent google-ing I'm not sure that it fits. I could be misunderstanding the term though. The only thing I assumed is that she actually wanted the meal that she had just told me she wanted. Which really isn't an assumption when someone comes up to a counter and says they want this meal.
However, her irritation was with me asking if she would like the dessert. I don't understand how "Would you like one?" assumed anything. Maybe I'm missing something.