From http://www.intersil.com/data/tb/TB389.pdf by Intersil titled PCB Land Pattern Design and Surface Mount Guidelines for QFN Packages
The PCB “thermal land” design for the exposed die pad
should include thermal vias that drop down and connect to
buried metal plane(s). This combination of vias for vertical
heat escape and buried planes for heat spreading allows the
QFN to achieve its full thermal potential.