Correct form: Я хочу чашку чаЮ, but if you want some tea you should to say: Я хочу чаЯ. If you mention about quantity tea or cognac you should use form чаЮ or коньякУ, if you don't tell about how many tea or cognac you want, you should use form чаЯ/коньякА

Correct form: Я хочу чашку чаЮ, but if you want some tea you should say: Я хочу чаЯ. If you mention about quantity tea or cognac you should use form чаЮ or коньякУ, if you don't tell about how many tea or cognac you want, you should use form чаЯ/коньякА.