Cream the butter. Add 5 tablespoons of icing sugar and mix well then stir in the vanilla. Gradually add the flour, mixing until fairly fluffy. Fold in the nuts. Shape into a roll (log). Wrap in greaseproof paper. Chill in the fridge for at least an hour. Cut into 3/8" to 1/2" slices and then in half. Roll into balls. Bake on an ungreased baking sheet for 12 to 15 minutes at 325F. While still warm dust lightly with the remaining tablespoon of icing sugar.