Bitter shortbread - what to do with it?
Based on my experience rescuing overdone oatcakes, I suggest crumbling up the shortbread, maybe cutting off any obviously burnt edges, adding some sugar and then using it as the topping for a fruit crumble. You could add another complex taste by adding crumbled nuts to the topping. (Mix them in, else they will burn during the baking.) There will probably be some bitterness remaining, but I'd expect this to mask it and make something palatable for most people.