I have a problem with my cookies becoming crumbly as they hang-out in room/warm afternoon temperatures.  I bake cookies -- a lot for fundraisers. Normally these cookies are stored in display cases within main halls and sold by students. What happens whenever I test them in the middle of the day, is that they've lost their tender toughness, which is more akin to a freshly baked batch or one from the refrigerator.  Since the cookies won't be sold through a refrigerated display-case for the foreseeable future, I wonder what's a good ingredient to make the cookies tougher within warm ambient temperatures? I suppose grocery-store cookies have this certain ingredient to a higher extent.  Thanks for your help!  The recipe is as follows  (Makes 120-130 cookies)  Wet:   1 lb butter (2x 220g), caramelized/browned to 300 F -- cooled 10 oz granulated sugar + 4 eggs + 4 tsp vanilla extract 10 oz brown sugar   Dry:   20 oz APF 6g baking soda, 8g iodized salt 2 tbsp corn starch 16 oz dark chocolate chips*   *I should disclose this. I use couverture chocolate chips so they're slightly melty, but also because they're cheaper than bakeproof ones. 
I'd imagine these are being stored in a simple glass / plastic display similar to what you'd see at a sari-sari store.   Shalryn's suggestion is good, but it compromises the quality of the cookie a bit. I'd look at using some sort of desiccant first, and a simple electric fan behind the display to move the air around in order to take advantage of it. Humidity and lack of air movement is definitely a big problem here, and if you can find a way to control that, you might not have to alter the recipe (which sounds positively delicious and I'm going to try it).  Some rice flour in a small shallow tin might just be enough to do it. You'd have to experiment a bit, and ultimately you might have to alter your recipe, but those cookies are surely a hit and I'd hate to see you have to make them harder for logistic's sake :)    If they're being sold in boxes, just add a silica gel pack and let folks know that they can't hit direct sunlight for too long. They are incredibly cheap (they run around 1.25 pesos from anywhere between 2-3 gram packets). 