Quite positive that it is not Shiitake

Looks to be some type of "bracket" forming fungus... I would have to assume that this species was already present, before you innoculated with your dowel spawn...
It is generally best to cut down fresh hardwood, at the end of Winter, when the sugar content is highest. Allow to rest until after all danger of frost is gone; taking care to not damage the bark. Once danger of frost is gone, innoculate ASAP!
Avoid direct contact with soil throughout process... If partially burying the log to facilitate moisture stabilization, backfill the hole with wet sand (free of debris).
Best wishes
