Using an electric mixer, cream the butter and sugar at low speed until it is smooth. Beat in the vanilla. At low speed gradually add the flour. Mix in the pecans with a spatula. With floured hands, take out about 1 tablespoon of dough and shape into a crescent. Continue to dust hands with flour as you make more cookies. Place onto prepared cookie sheets. Bake for 40 minutes. When cool enough to handle but still warm, roll in additional confectioners' sugar. Cool on wire racks.