when is ice cream countable and when uncountable?

[CarloSsS]

I found these two example sentences in a dictionary:

Desserts are served with cream or ice cream.
Who wants an ice cream?

What puzzles me is, how is the ice cream in the first sentence different from the ice cream in the second one? I think that in the first sentence, ice cream is referred to in general (that is some frozen cream with fruit), while in the second sentence ice cream is considered a portion of ice cream perhaps? Such as a cone of ice cream, or three scoops of ice cream in a bowl or in some other type of a container?

 

[abaka]

Without the indefinite article, you are talking about ice cream as a substance, a generic kind of food.
The indefinite article denotes a (a[n] = one) serving of ice cream: one cone, one sundae, one bowl, etc.

 

[SoothingDave]

I don't think I would refer to "an" ice cream unless it was a prepackaged serving.