I posted one of these a while back but didn't get the underside. I have managed to do that today. I have also found others that are similar but a different colour. As I am not sure if they are the same or not, I will post the others as another sighting. It is worth noting though that the first time I sighted these there were some small and some big but they remained white in colour.
A great looking Amanita species. I need to go and check on my literature to see if I can come up with a species name for you. I'll have a look over the next few days hopefully!