Cream together the butter with the sugar in a large mixing bowl. Beat in egg yolks and vanilla. Gradually beat in flour.
Turn into a ball, cover and let stand at room temperature for 2 hours.
Preheat oven to 375 degrees F (190 degrees C).
To form the crescents, roll about 1 Tbsp of the dough in the palms of your hands. Then continue rolling to form a rope about 3 inches long and 1/2 inch thick. Bend rope to form crescent. Repeat with remaining dough, placing on ungreased cookie sheets about 1 inch apart.
Bake for about 10 to 12 minutes. Cookies should be white in color.
Let cool on cookie sheets for 5 minutes to prevent breaking. Then, while cookies are still warm, carefully roll in vanilla sugar. (May be found in bulk food stores). Repeat with remaining cookies. Cool cookies completely.
Melt chocolate in a double boiler set over low heat. When cookies are cool, dip tips of each cookie in melted chocolate, then set on waxed paper to dry.
