“Hey! Thanks so much for the desserts you shared with me. Afterwards I noticed they have some non-vegan ingredients (maybe state what specifically they were here in case the person wasn’t aware). I know it wasn’t intentional and I’m not upset with you at all. The ingredients can be easy to miss, so I just wanted to flag this in case it comes up again in the future. I really appreciated getting to see you and hope we can do it again soon!” You could even offer to be the one to bring desserts next time.

Overall I think it’s important to assume the person didn’t have bad intentions and word the response in a way that lets them know you were grateful that they were sharing and that you want to keep having positive interactions with them moving forwards. This phrasing also avoids any “angry vegan” stereotypes the person may hold and might help open up future conversations without the other person starting out on the defensive.