Posted 12/11/2012 5:53:19 AM
I've been making these for almost 35 years, since 2nd grade! The only times they've come out crumby or dry are when I used an electric oven at a relative's house, and when I didn't combine the dough enough. I can't imagine what adding an egg would do, as the previous user tried. It would probably change the character of the cookie, making it cake. It's essentially a shortbread cookie (no egg, lots of butter). They should come out dense, buttery and melt in your mouth. I always say these are "made of sugar, butter and love."
I also find it helps to pulse the powdered sugar in a food processor to get out the lumps, then add the nuts and pulse a bit more. It seems to bring out the oil in the nuts, which helps with the moisture. If you don't have a food processor, a blender would also work. I think even a quick chop will help release the oils from the nuts.