My answer is the same almost every time there is a dinner served. If dinner is being served, then dessert should come after the Fundraising portion of the evening. Or if the meal is being served family style - do it during the Live Auction. Ensure that your guests aren't starving when you start asking for money. A hungry crowd isn't a fun crowd to work with.
I love conducting the Live Auction during dinner. This way we don't waste any time and are getting the money early. We don't want guests and potential donors to be too tired, to drunk or too concerned about the babysitter before bidding and donating.
So serve dessert after you've reached your fundraising goal and use it as a celebratory moment.