In a large mixing bowl, cream together the butter & granulated sugar, then beat in the egg yolks & vanilla extract, before gradually beating in the flour.
Form the dough into a ball & let it stand at room temperature for 2 hours.
Preheat oven to 375 degrees F.
To form the crescents, roll about 1 tablespoon of the dough in the palms of your hands, continuing to roll & form a rope about 3 inches long & 1/2 inch thick. Bend the rope to form a crescent, then repeat with the remaining dough, while placing the crescents on ungreased cookie sheets about 1 inch apart.
Bake about 10-12 minutes (Cookies will STILL be white in color.), then let them cool on the cookie sheets for 5 minutes to prevent breaking.
While cookies are still warm, carefully roll each one in vanilla sugar, before letting them cool completely.
In a double boiler, melt chocolate over low heat, & when cookies are cool, dip one tip of each cookie into the melted chocolate, then set on waxed paper to dry.