1) Find where the cornice is thinnest and overhangs the least
2) Have someone establish a protected belay well out of the fall line, belayed to a big ass bollard, buried deadman, etc.
3) Take your adze and get to digging. Cut away everything overhanging above you. Basically make a vertical trench and stem your way up it until you can reach higher.
4) Once you cut your way through, belay down the far side where you aren't gonna go for a ride if the cornice detaches suddenly.