it's what do you eat in the morning. ( coffe, milk, cake, eggs....)

Hi. The adverb form of fast is irregular. It is one of several exceptions. So to add an " ly " after every adjectives it's not a rule. for exemple : the adverb of good is't goodly, is "well". other exemple : a fast runner / run fast a hard worker / work hard a bad smell / smell bad etc...