In reply to The Ice Doctor:
No, it's a bit easier than you think. If you can get to Chamonix, then there's a daily bus service through the tunnel that takes you to the bus station in Aosta centre. From there, there's a regular bus service up to Cogne. If you need to, you can then get to Lillaz or Valnontey by either bus or taxi.
A bit of googling will get you the times etc.