Preheat oven to 325F. Beat the butter, sugar and vanilla with an electric mixer for a few minutes, until pale and creamy. Beat in the nuts and flour. Roll into walnut-sized balls and place an inch or so apart on an ungreased cookie sheet. Bake for 20-25 minutes, until pale golden on the bottoms. Meanwhile, put some icing sugar into a shallow dish; when the cookies come out of the oven, roll them in icing sugar to coat while still warm. (The icing sugar mingles with the butter in the cookie, coating them with a sort of buttery, nutty icing.) Makes 2 1/2 dozen cookies.