Learn Swedish: Vi brukar doppa bakverket i kaffet. - We usually dip the pastry in the coffee.

You would say Vi brukar doppa ett slags bakverk i kaffet or Vi brukar doppa en sorts bakverk i kaffet. Remember that en sorts and ett slags have fixed genders based on the nouns sort (en-word) and slag (ett-word). They don't change to match the noun they describe, so both phrases are correct here regardless of bakverk being an ett-word.

In the English translation, 'brukar' is translated as 'usually'. Is it an adverb in Swedish?

No, brukar is an actual verb! English lacks a direct equivalent for the verb 'to be in the habit of doing something', so we translate it with the adverb 'usually'. Because it is a verb, it changes tense: for example, vi brukade doppa would mean 'we used to dip'.

Why isn't there an 'att' (to) between 'brukar' and 'doppa'?

Because brukar functions as an auxiliary (helping) verb. Just like kan (can), vill (want), and måste (must), it is followed directly by the bare infinitive of the main verb, so we leave out att.

How do we know if 'bakverk' is an 'en' or 'ett' word?