The nut traditionally used for this cookie is almond. I decided I needed something different this year, so I used cashew nuts and I believe the result was more tasty. The big secret to making successful melt-in-the-mouth "kourambies" is to beat in a kitchen machine the butter with the icing sugar for 15-20 minutes. It might seem too long but it really makes the difference as it makes the butter fluffy. The second secret is that the taste of the final cookie depends mostly on the quality of the butter. I use a good quality non-smelly and I avoid totally margarine or substitutes. I also noticed that by using traditional yellowish flour (the one we use for kneading bread)instead of all-purpose one, the results are much better.