I had found a very cool product last year called the Backcountry Bomb. It's a device specifically made to cut cornices. I decided to make my own. Here's how:
First you will need to head down to Lowe's or Home Depot or similar. Pick up 41 feet of 1/16 uncoated wire, one bag of 1/16 swage/stopper fittings (there's X2 per bag), and two 1/2 X 1/2 PVC Tees.
Then gather the following complex tools; hammer, blunt chisel (or screwdriver you don't like), pliers, duct tape.
Feed the wire into the stopper and then put on the swage. Fit it so it makes it around the 1/2 pipe of the tee. Have about 1/4 inch of wire sticking out from the bottom of the swage.
Crimp it with the pliers to hold it in place while you get your swage tool. If you don't have a swage tool, a blunt chisel or old regular screwdriver will work.
Crimp the swage with the chisel or swage tool.
Butt the stopper up to the wire and flatten it with the hammer. The swage should have nice, neat indentations on both sides. Repeat with the other end of the wire.
Feed the loop up through the bottom of the Tee and out one side, then looped over the other. The stopper should catch the edge when pulled from the bottom. This helps give it more strength instead of just pulling on the loop. This is how the cutter will look when you actually use it. Grasp it like you are starting a lawnmower.
Now paint the handles a nice manly color so the cornice knows you mean business. Also it should be bright and not white since you may drop one in the snow. Duh! Push a loop through the side and poke it through the bottom and then into the other handle's bottom and out the side. Place the Tee's together and start winding the wire around as see below. DO NOT wind it like you used to wind extension cords when you were a kid and your Dad yelled at you. Instead, turn the Tee's (actually, now an H) end for end and feed the wire on. It will wind up and deploy a lot easier if you always do it that way.
It is about the size of a sandwich and weighs in at around 7 ounces. If I wasn't sure that I was going to encounter cornices, I would rather pack another turkey sandwich. If you are going to encounter cornices that you want to cut to test a slope you want to ski and you absolutely KNOW there's nobody below you, it's worth the weight.
The easiest way to use it is to probe the shit out of the top to make sure you aren't where it will fracture just by standing around.
Then use a keychain carabiner to clip a loop (minus the handle) to your probe's top loop and sling it around the cornice from a distance. It's probably not a bad idea to rope up, either. Once around the cornice, start sawing. Two people works well, but one is fine and the other person can manage the belay (if used). Drop the cornice and observe what the slope does. If you drop a 900 pound cornice down a chute and nothing happens, it's pretty safe to say that you can ski it! Just don't get cut-happy and scour chutes of their snowpack before they can stabilize.
I'm coming for you buddy!