In large mixer bowl, beat butter with electric mixer for 30 seconds. Add powdered sugar; beat until fluffy. Beat in the egg yolks and vanilla. Stir in the flour. Cover and chill 1 hour. Work with a little dough at a time; keep the remainder chilled. Using about 1 tablespoon dough for each cookie, roll into 6" long ropes. Shape each into a wreath, overlapping about 1" from ends. Brush each cookie with egg white; sprinkle with some crushed sugar Place on a ungreased cookie sheet. Bake in 325 degree oven for 15 to 17 minutes or until cookies are golden. Cool 1 minute. Remove cookies to a wire rack; cool.