"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.

All your examples work. countable: "Excuse me, how many ice creams can I buy for $10?" "Five ice creams." uncountable: "There is not enough ice cream for everyone."

"Mummy, can I have some ice cream?" There is a two-litre tub of ice cream in the freezer. The child wants the mother to get a spoon or a scoop, and to take some ice cream out of the tub and serve it in a dish for him to eat. He obviously can't eat the whole tub of ice cream - he just wants SOME of it. This is an uncountable use of the word 'ice cream'. "Mummy, can I have an ice cream?" There is a pack of six individually wrapped ice cones in the freezer. He want ONE of them. This is a countable use of the word "ice cream".

Think about it like this: you can drink some water or you can drink a glass if water. An ice cream is like a glass of water. It is a unit of measure. It can be a dish (containing 6 ounces, for example), or an ice cream cone (containing 3 ounces of ice cream, for example). Ice cream, without "an" or a number like 2 or 3, is the food you will eat, regardless of how much. Ice cream, cake, meat, corn. With each of these foods, I haven't told you how much.

Ice cream is uncountable. You can't have many ice cream. You would say scoop of ice cream, bowl of ice cream, gallon of ice cream. If you need to specify how it is served (slice, piece, etc.) it is uncountable.