Learn Croatian: Želim dvije kuglice vanilije u čašici. - I want two scoops of vanilla in a cup.

The number two changes based on the gender of the noun it counts. Because kuglica (scoop/small ball) is a feminine noun, you must use the feminine form dvije. If you were ordering something masculine, like two ice creams, you would use dva (dva sladoleda).

What case is kuglice in, and why?

It is in the genitive singular. In Croatian, the numbers 2, 3, and 4 require the following noun to be in the genitive singular (sometimes called the paucal form). For feminine nouns ending in -a, this form happens to look exactly like the standard nominative plural, but grammatically it is genitive singular.

Why is the flavor vanilije in the genitive case instead of being an adjective?

While English uses an adjective or noun adjunct like 'vanilla scoops', Croatian literally says 'scoops of vanilla' (kuglice vanilije). You will see this genitive noun construction used for almost all ice cream flavors, such as kuglica čokolade (a scoop of chocolate).

Why use čašici instead of čaši?

Čašici is the locative form of čašica, which is the diminutive of čaša (glass). Ice cream shops serve scoops in small paper or plastic cups, so the diminutive čašica is the most natural word to use here rather than the word for a full-sized drinking glass.