Learn Croatian: Želite li ukusan kolač? - Do you want a tasty cake?

Because kolač (cake) is a masculine inanimate noun (a non-living thing). In Croatian, the accusative case for masculine inanimate nouns and their adjectives is exactly the same as the dictionary form (the nominative). If you were talking about a living thing, the endings would change.

Why is the word li placed right after the verb?

The word li is a tiny particle used to make yes/no questions. When you build a question this way, the verb must come first, followed immediately by li. You might also hear people say Da li želite ukusan kolač? which uses a different structure but means exactly the same thing.

Does želite mean you are offering cake to one person or to a group of people?

It could be either! The verb ending -te is the plural "you", so you use it when offering cake to a group. However, Croatian also uses this exact plural form to be polite and formal to a single person, like a customer in a cafe or someone you just met.

If the adjective is ukusan, why do I sometimes see it spelled as ukusna in other sentences?

The "a" before the "n" in ukusan is what we call a "fleeting a". It gets inserted into the masculine dictionary form to make the word easier to pronounce. When you add endings for other genders, like the feminine -a, that extra "a" drops out, making ukusna instead of "ukusana". Here, because masculine inanimate accusative copies the dictionary form, the "fleeting a" stays!