Good usage: express what you are good at. At can be used as a preposition here in three ways: one is the noun sth, and the other is the pronoun it, that, this and so on. Third, V+ing gerund (when you want to express what you are good at, you need to add ing here to become its corresponding gerund form.

For example, being good at singing gerund. Do something, a verb, just like what you wrote about cooking, so you need to add ing after cook to become its gerund form of cooking.

Be good at English nouns

Good at it pronouns, good at it.