Pre-heat the oven to 140 C / Gas 2. Grease a baking tray.
In a bowl cream together the sugar and butter. Add the egg and vanilla and beat in.
Incorporate the flour and blend well together. Fold in the pistachios.
Shape into balls about the size of a golf ball and place onto the baking tray. Using the palm of your hand flatten out (not too thin).
Bake for 20 to 22 minutes. The cookies are cooked when the top is firm. (Don't worry if the centre is slightly soft as the cookies will harden when cooled). Place onto a wire rack to cool.
Tip
Depending on the size of your baking tray, you may need to do in two batches.