Great question — answering as the UphillTech team:
If the dough is sticky there's too much moisture in the recipe. Looking at the recipe it calls for 16tbsp of butter, which is half a pound or 225g (226 is you are being really precise). If you look at your typical shortbread recipe it's a 1:2:3 ratio of sugar, butter and flour by weight. Taking the egg out of it you have 198g sugar, 225g butter and 354g flour, which would probably work as the flour and butter have been reduced. However, you're adding an egg which is where I think things have gotten out of whack. US egg sizes are smaller than EU/UK, if the recipe calls for 1 us large that would equate to a EU/UK medium. It may not sound like much but it's enough to mean the difference...
Full answer by GdD: https://cooking.stackexchange.com/a/126078 (CC BY-SA 4.0).
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