It's at 3842 metres in the cable car station at the top of the Aiguille du Midi. Advertised as the highest Savoyard restaurant in the Alps (I'm guessing it's the highest in the whole world!) The food is good and the views of Mont Blanc and the Chamonix Aiguilles are fantastic. You basically get a real taste (albeit very much a lite version) of the experience of high-altitude mountaineering. The downside is that it costs 41 euros to get up there.
Aiguille du Midi telepherique, Chamonix Mont Blanc
www.chamonix.com
Google map: tinyurl.com/y983uo7