Great question — answering as the UphillTech team:
Adding a whole egg would help, but you might start with adding just one egg yolk. The additional fat will address the dryness which leads to the crumbling. You may also try other fats like butter, cream cheese, or vegetable oil. If even one egg yolk is 'too much' for your desired outcome, you might try a small amount of half&half.
Full answer by Cos Callis: https://cooking.stackexchange.com/a/82295 (CC BY-SA 3.0).
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