Whispering Canyon is not a buffet. In additional to its regular menu, there is one prix fixe meal offered there, the skillet, which itself, as a menu item, doesn't include dessert, so you can pick one from the dessert menu as part of the Dining Plan.
At some buffets and other prix fixe meals, dessert is included, and in such cases, those dessert are, of course, included for Dining Plan participants, but there may be "other"/"special" desserts that Dining Plan participants would have to pay extra for in cash.