The batch I described makes three 8x8" pan's worth (so there's only 1/3 of the dough in the pan). The edges would crisp up after cooling, but that's it. So tonight I took your suggestions and checked Rose Levy Berenbaum's recipe, too. I baked another pan, with the dough a bit thinner (3/8"), at 275ºF for an hour & 10 minutes. It worked! Thanks!

You didn't mention what stage of the cooking you're at.... I always find that the shortbread crisps up as it cools - that's why it needs to be cooled on the pan.

Is all that dough for one pan?? Could be having a crisping problem because so thick. For that amount of dough, I'd use maybe four 8" tart pans. Will make a thinner, crisper shortbread.

I agree with Soozll, bake lower temp, longer time.

seems like a lot of dough for an 8"pan and the low temp for only 25-30 mins might also affect the texture. I'd bake it longer at a lower temp, maybe 50 minutes or so at 300/F