Indefinite article vs numeral: use of "一" (yī)
I think ice cream is probably not the best example to demonstrate this, since it comes in many forms and some are counted as one ice cream (e.g. those that are attached to a stick), others as some ice cream (those that come in a huge platic box and you need to create portions and serve them to your family members). So I'll use a different example, something that is fully countable, 毛衣 (sweater or pullover).

If you want to say that you would like to buy a sweater, in Chinese you have three options.


我想买毛衣。No classifier/measure word, no number, just the bare noun. In this case the number of the sweaters is fully indefinite.
我想买件毛衣。No number, only the measure word + noun. Here 一 is dropped for the sake of brevity, but tacitly you always mean one item. This is the best way to use if you would like to say "a sweater".
我想买一件毛衣。Number + measure word + noun. This corresponds the best to "one sweater".