When I doubled my last batch of peanutbutter cookie dough I only added one egg instead of two. Now it is crumbly and won't form a ball. Can I add another egg now? Or is adding some water better?
You can, but you should take care to not overwork the dough when you add the final egg. Whisk the egg lightly on its own and then mix it in as quickly as possible to avoid developing too much gluten in the dough. You will need the binding power of the egg for the cookies to hold together well after baking. Water will not help with that.