To avoid freezing your bum off while sitting down in snow or on cold ground while winter camping, you'll want to use some sort of foam pad. For instance, if you are winter camping and have brought along a foam sleeping pad like the Therm-A-Rest Z-Lite SOL Sleeping Pad, you can use that to sit on top and protect your butt from the cold.
Alternatively, Therm-A-Rest also makes various trail seat pads to sit on:
However, you can always just sit on your own backpack or jacket... just make sure you don't have anything breakable inside the pack (stoves, water bottles, sunglasses, goggles, etc.) before sitting down.
Lastly, you could always stack up pine bows to help insulate your butt from the cold ground.