Too late now, obviously, but I can think of four things.
In ascending order of formality:
1. Joe's Italian Ice, Harbor Blvd. south of Chapman Ave., Garden Grove: I am in love with the Joelatti, which is Italian ice and soft-serve together in one cup. It's a shack; there are maybe four bar stools on the north side of the shack.
2. Golden Spoon, Katella Ave. (Costco plaza), Cypress (among other locations): peanut butter frozen yogurt, that's all I have to say about that.
3. Thai Nakorn, Beach Blvd. and Chapman Ave., Stanton: besides the very good mango and sticky rice, they have all kinds of Thai desserts, including the required pandan-coconut combination.
4. The Catch, State College Blvd. and Gene Autry Way, Anaheim: this is the sister restaurant of Taps in Brea, and they have both bananas Foster and this incredibly decadent "volcano" cake that is just out of this world.