Cut out any buds with rot and throw them away (do not smoke), its a disease, and it will only get worse and can easily take out the whole crop. The problem is caused by high Rh and that needs to be gotten under control ASAP, or it will only continue to happen. A spray of beneficial bacteria would help slow the problem down. Also a sulfur burner is a must in a grow room, this will also help, as a preventive measure.