Umm sorry. There is NO legal requirement to use an independent, or ‘special’ prosecutor in this situation. Unless you could point me to an authority that states otherwise?
Specifically, look up Rule 1.7 regarding conflicts of interest related to current clients ... keeping in mind that for Jeff Sessions, his "current client" is the United States of America, not Donald Trump.
This is why the stories about Trump wanting Sessions not to recuse himself were probably true ... because Trump probably has no idea about how these ethics rules apply to a situation that he is always seeing (understandably) in political terms.