"Ice cream" can certainly be either uncountable and countable.
Your scenario of someone holding two ice creams is a good example of a countable use. In each hand, the person has a cone with a scoop of ice-cream on the top. The ice-cream on the top is uncountable, but the SINGLE ITEM that he has in his hand ( the sum total of a cone, a portion of ice cream, maybe some chocolate sprinkles and some raspberry sauce) could certainly be called AN ICE CREAM. He has two ice creams (plural and therefore countable) and he will give you one ice cream (also countable).
Imagine going into an ice-cream shop. There are large vats of different flavours of ice cream. You might ask for some vanilla and some strawberry ice cream, because at this point the ice cream is 'stuff' (uncountable). It's measured in litres and scoops. But as soon as it's in your hand in a cone, it's fine to call it 'an ice cream', because it's one individual 'thing'. [ Some people find the distinction between 'stuff' and 'things' easier to grasp than countable/uncountable].
Or think of the ready-made products that you can buy : individual portions of ice cream on a stick, covered in chocolate and nuts or whatever, sold in a plastic wrapper. These products are obviously 'things' aren't they? And it's fine refer to them as ice creams (plural) or an ice cream (singular).
The other interpretation of 'an ice cream' (countable) is when this means 'a type of ....'. If you say 'An ice cream that I really love is ginger and honey'. The same goes for most uncountable nouns - for example, 'flour' is uncountable, but a baker or pasta maker would be aware there are different types of flours (countable).
I hope that helps.