Whatever you do don't try to use an easy out. I have rarely read of anyone having success with them, the vast majority have broke them off and that only makes the problem worse.The best way is to weld a nut onto the broken off bolt and let it cool before you try to turn it out. I have had good luck drilling an appropriate sized hole in a short piece of flat iron and then welding that to the bolt end. Make it long enough to work as it's own wrench.
The way the welding works to help loosen the bolt end is because when it's heated to such a high temperature, it expands from the heat and since the metal around the bolt doesn't reach near as high temperature as the bolt end and doesn't expand very much, the bolt can't expand radially so it expands in an axial direction. When it cools, it is free to contract in a radial direction so it loosens in the hole. That is why it is important to get lots of heat into it when you weld it and important to let it cool properly before you try to turn it out. Having a cup of coffee is a good way to time the cool down period.