OK, so there are no private dessert parties inside Disneyland. DCA has them, but you have to have the desserts in one place and then move to the standing room-only spot in the World of Color viewing area. They just started a public dessert party that actually serves the desserts in front of the show, but private events are still done separately. You could also do a non-fireworks dessert party anywhere at the resorts. But it's just, like, standing around eating desserts.
The only way you could do something like that in your wedding clothes is to add on an after hours private viewing of World of Color to your wedding. From what I have heard, it is extremely expensive to do this and I'm not 100% sure if it is still even an option.