Take an old junk allen wrench that is smaller than the diameter of the bolt (remainder). Carefully JB Weld it to the top of said bolt. Come back tomorrow and turn the allen wrench and your broken bolt should come right out with it.

I would like to offer a POV that has not been presented. Are you sure the issue is from overtightening, and not undertight?

Here is why I ask. I walk around on Titanium all day long, my leg's pylon is made of it. When the bolts aren't tightened properly (tight enough) it causes lateral stresses that the head should not encounter and will cause them to snap over time. My prosthesist replaces all the pylons and clamps on my leg at least once every two years, just to be sure they don't fail. The only time I have had one break was from a toe adjustment that I made and didn't tighten down hard enough afterward.
Of course, what I walk on, and what that stem bolt was are very likely a world apart.