The dessert has to be ordered/purchased at the same time as the rest of the meal. As POOHsie said, some of the resort food courts offer ice cream. You will have to let the cashier know this is what you have chosen at the time you purchase the entree and beverage, but they'll stamp your receipt and you'll be able to come back (usually within an hour) to get the ice cream.
Some desserts can be snack credit items, but since you're using a QS credit the cashier will know it is part of the QS credit.
At the resort food courts and QS places you can ask the cashier or CMs behind the counter what is included in the entree, dessert and beverage. The beverage is usually the standard fountain drinks such as soda, coffee, tea, iced tea, lemonade. (At many QS places you can get a bottle of water in lieu of a dessert).