Outside we tried everything from street food to simply food court fare to restaurants. Heck, there's even McDonald's. As I've said before I try out all the different stuff on the McDonald's menu when we travel.
There's the Cream & Fudge Factory at Siam Paragon (they also have this at the Suvarnabhumi Airport). It's not unique where you concoct your own combinations, but the ice cream is great.
Chocolate Truffle Sensation 95 baht