The best method to store buffy coats is frozen at -80°C.
Mychaleckyj, J. C., Farber, E. A., Chmielewski, J., Artale, J., Light, L. S., Bowden, D. W., ... & Marcovina, S. M. (2011). Buffy coat specimens remain viable as a DNA source for highly multiplexed genome-wide genetic tests after long term storage. Journal of translational medicine, 9(1), 91.