You will need more heat than the standard propane torch, get the mapp gas torch. Patience is the key to success, and not burning yourself. You might consider removing that side of the stern bracket so you can clamp in a a big vise. Having an assistant helps as well. Let the assistant apply the heat while you apply pressure to the vise grip, hoping for even the slightest movement of the snapped screw. Once it moves slightly, let it cool, then repeat the process until it moves some more. In this case you have that nice little valley between the bracket and screw to add oil. Be careful with the heat near the thin part of the casting, don’t melt it.
